diff --git a/src/wp-includes/ms-blogs.php b/src/wp-includes/ms-blogs.php
index cdb9691b90..ab98d6e5cb 100644
--- a/src/wp-includes/ms-blogs.php
+++ b/src/wp-includes/ms-blogs.php
@@ -70,23 +70,38 @@ function get_blogaddress_by_name( $blogname ) {
 }
 
 /**
- * Retrieves a site's ID given its (subdomain or directory) slug.
+ * Retrieves a sites ID given its (subdomain or directory) slug.
  *
  * @since MU (3.0.0)
  * @since 4.7.0 Converted to use get_sites().
- * @since 4.9.0 Converted to use get_site_by().
  *
  * @param string $slug A site's slug.
  * @return int|null The site ID, or null if no site is found for the given slug.
  */
 function get_id_from_blogname( $slug ) {
-	$result = get_site_by( 'slug', $slug );
+	$current_network = get_network();
+	$slug = trim( $slug, '/' );
 
-	if ( ! $result ) {
+	if ( is_subdomain_install() ) {
+		$domain = $slug . '.' . preg_replace( '|^www\.|', '', $current_network->domain );
+		$path = $current_network->path;
+	} else {
+		$domain = $current_network->domain;
+		$path = $current_network->path . $slug . '/';
+	}
+
+	$site_ids = get_sites( array(
+		'number' => 1,
+		'fields' => 'ids',
+		'domain' => $domain,
+		'path' => $path,
+	) );
+
+	if ( empty( $site_ids ) ) {
 		return null;
 	}
 
-	return $result->id;
+	return array_shift( $site_ids );
 }
 
 /**
@@ -528,118 +543,6 @@ function get_site( $site = null ) {
 	return $_site;
 }
 
-/**
- * Retrieves a site by a given field and value.
- *
- * @since 4.9.0
- *
- * @param string     $field      Name of a field to query against. Accepts 'id', 'slug', 'url',
- *                               'domain' (if a subdomain install) or 'path' (if a subdirectory install).
- * @param string|int $value      The search value for $field.
- * @param int|null   $network_id Optional. ID of the network. Default is the current network.
- * @return WP_Site|null The site object or null if not found.
- */
-function get_site_by( $field, $value, $network_id = null ) {
-	$args = array();
-
-	// `get_sites()` will return unexpected results if empty strings are passed as arguments.
-	if ( 'slug' === $field || 'url' === $field || 'domain' === $field || 'path' === $field ) {
-		if ( ! is_string( $value ) && ! is_numeric( $value ) ) {
-			return null;
-		}
-
-		$value = trim( $value );
-
-		if ( 0 === strlen( $value ) ) {
-			return null;
-		}
-	}
-
-	switch ( $field ) {
-		case 'id':
-			if ( ! is_numeric( $value ) ) {
-				return null;
-			}
-			$args['site__in'][] = intval( $value );
-			break;
-		case 'slug':
-			$network = get_network( $network_id );
-			if ( ! $network ) {
-				return null;
-			}
-
-			if ( is_subdomain_install() ) {
-				$args['domain'] = trim( $value, '/' ) . '.' . preg_replace( '|^www\.|', '', $network->domain );
-				$args['path'] = $network->path;
-			} else {
-				$args['domain'] = $network->domain;
-				$args['path'] = $network->path . trim( $value, '/' ) . '/';
-			}
-			break;
-		case 'url':
-			if ( 0 !== strpos( $value, 'http://' ) && 0 !== strpos( $value, 'https://' ) ) {
-				$value = 'http://' . $value;
-			}
-
-			$parts = wp_parse_url( $value );
-			if ( ! $parts ) {
-				return null;
-			}
-
-			$args['domain'] = $parts['host'];
-			if ( ! empty( $parts['path'] ) ) {
-				$args['path'] = '/' . trim( $parts['path'], '/' ) . '/';
-			} else {
-				$args['path'] = '/';
-			}
-			break;
-		case 'domain':
-			if ( ! is_subdomain_install() ) {
-				return null;
-			}
-
-			$args['domain'] = $value;
-			break;
-		case 'path':
-			if ( is_subdomain_install() ) {
-				return null;
-			}
-
-			$args['path'] = '/' . trim( $value, '/' ) . '/';
-			break;
-		default:
-			return null;
-	}
-
-	if ( isset( $args['domain'] ) && substr( $args['domain'], 0, 4 ) === 'www.' ) {
-		$nowww = substr( $args['domain'], 4 );
-
-		$args['domain__in'] = array( $nowww, $args['domain'] );
-		unset( $args['domain'] );
-
-		$args['orderby'] = 'domain_length';
-		$args['order']   = 'DESC';
-	}
-
-	if ( isset( $args['path'] ) ) {
-		$args['path'] = str_replace( '//', '/', $args['path'] );
-	}
-
-	if ( ! empty( $network_id ) ) {
-		$args['network_id'] = (int) $network_id;
-	}
-
-	$args['number'] = 1;
-
-	$sites = get_sites( $args );
-
-	if ( empty( $sites ) ) {
-		return null;
-	}
-
-	return array_shift( $sites );
-}
-
 /**
  * Adds any sites from the given ids to the cache that do not already exist in cache.
  *
diff --git a/tests/phpunit/tests/multisite/getSiteBy.php b/tests/phpunit/tests/multisite/getSiteBy.php
deleted file mode 100644
index 1ff5c455e1..0000000000
--- a/tests/phpunit/tests/multisite/getSiteBy.php
+++ /dev/null
@@ -1,252 +0,0 @@
-<?php
-
-if ( is_multisite() ) :
-/**
- * Test get_site_by() in multisite.
- *
- * @ticket 40180
- * @group ms-site
- * @group multisite
- */
-class Tests_Multisite_Get_Site_By extends WP_UnitTestCase {
-	protected static $network_ids;
-	protected static $site_ids;
-
-	public static function wpSetUpBeforeClass( $factory ) {
-		self::$network_ids = array(
-			'wordpress.org/'         => array( 'domain' => 'wordpress.org',     'path' => '/' ),
-			'www.wordpress.net/'     => array( 'domain' => 'www.wordpress.net', 'path' => '/' ),
-		);
-
-		foreach ( self::$network_ids as &$id ) {
-			$id = $factory->network->create( $id );
-		}
-		unset( $id );
-
-		self::$site_ids = array(
-			'wordpress.org/'              => array( 'domain' => 'wordpress.org',     'path' => '/',     'site_id' => self::$network_ids['wordpress.org/'] ),
-			'foo.wordpress.org/'          => array( 'domain' => 'foo.wordpress.org', 'path' => '/',     'site_id' => self::$network_ids['wordpress.org/'] ),
-			'wordpress.org/foo/'          => array( 'domain' => 'wordpress.org',     'path' => '/foo/', 'site_id' => self::$network_ids['wordpress.org/'] ),
-			'www.wordpress.org/'          => array( 'domain' => 'www.wordpress.org', 'path' => '/',     'site_id' => self::$network_ids['wordpress.org/'] ),
-			'www.wordpress.net/'          => array( 'domain' => 'www.wordpress.net', 'path' => '/',     'site_id' => self::$network_ids['www.wordpress.net/'] ),
-			'foo.wordpress.net/'          => array( 'domain' => 'foo.wordpress.net', 'path' => '/',     'site_id' => self::$network_ids['www.wordpress.net/'] ),
-			'www.wordpress.net/foo/'      => array( 'domain' => 'www.wordpress.net', 'path' => '/foo/', 'site_id' => self::$network_ids['www.wordpress.net/'] ),
-		);
-
-		foreach ( self::$site_ids as &$id ) {
-			$id = $factory->blog->create( $id );
-		}
-		unset( $id );
-	}
-
-	public static function wpTearDownAfterClass() {
-		global $wpdb;
-
-		foreach( self::$site_ids as $id ) {
-			wpmu_delete_blog( $id, true );
-		}
-
-		foreach( self::$network_ids as $id ) {
-			$wpdb->query( $wpdb->prepare( "DELETE FROM {$wpdb->sitemeta} WHERE site_id = %d", $id ) );
-			$wpdb->query( $wpdb->prepare( "DELETE FROM {$wpdb->site} WHERE id= %d", $id ) );
-		}
-
-		wp_update_network_site_counts();
-	}
-
-	public function test_get_site_by_id_with_valid_id() {
-		$result = get_site_by( 'id', self::$site_ids['wordpress.org/'] );
-
-		$this->assertEquals( self::$site_ids['wordpress.org/'], $result->id );
-	}
-
-	public function test_get_site_by_id_with_invalid_id() {
-		$result = get_site_by( 'id', 'wp.org' );
-
-		$this->assertNull( $result );
-	}
-
-	public function test_get_site_by_slug_subdomain() {
-		if ( ! is_subdomain_install() ) {
-			$this->markTestSkipped( 'This test is only valid in a subdomain configuration.' );
-		}
-
-		$result = get_site_by( 'slug', 'foo', self::$network_ids['wordpress.org/'] );
-
-		$this->assertEquals( self::$site_ids['foo.wordpress.org/'], $result->id );
-	}
-
-	public function test_get_site_by_slug_with_www_subdomain() {
-		if ( ! is_subdomain_install() ) {
-			$this->markTestSkipped( 'This test is only valid in a subdomain configuration.' );
-		}
-
-		$result = get_site_by( 'slug', 'foo', self::$network_ids['www.wordpress.net/'] );
-
-		$this->assertEquals( self::$site_ids['foo.wordpress.net/'], $result->id );
-	}
-
-	public function test_get_site_by_slug_subdirectory() {
-		if ( is_subdomain_install() ) {
-			$this->markTestSkipped( 'This test is only valid in a subdirectory configuration.' );
-		}
-
-		$result = get_site_by( 'slug', 'foo', self::$network_ids['wordpress.org/'] );
-
-		$this->assertEquals( self::$site_ids['wordpress.org/foo/'], $result->id );
-	}
-
-	public function test_get_site_by_slug_with_www_subdirectory() {
-		if ( is_subdomain_install() ) {
-			$this->markTestSkipped( 'This test is only valid in a subdirectory configuration.' );
-		}
-
-		$result = get_site_by( 'slug', 'foo', self::$network_ids['www.wordpress.net/'] );
-
-		$this->assertEquals( self::$site_ids['www.wordpress.net/foo/'], $result->id );
-	}
-
-	public function test_get_site_by_slug_with_first_network() {
-		$result = get_site_by( 'slug', 'foo', self::$network_ids['wordpress.org/'] );
-
-		if ( is_subdomain_install() ) {
-			$this->assertEquals( self::$site_ids['foo.wordpress.org/'], $result->id );
-		} else {
-			$this->assertEquals( self::$site_ids['wordpress.org/foo/'], $result->id );
-		}
-	}
-
-	public function test_get_site_by_slug_with_second_network() {
-		$result = get_site_by( 'slug', 'foo', self::$network_ids['www.wordpress.net/'] );
-
-		if ( is_subdomain_install() ) {
-			$this->assertEquals( self::$site_ids['foo.wordpress.net/'], $result->id );
-		} else {
-			$this->assertEquals( self::$site_ids['www.wordpress.net/foo/'], $result->id );
-		}
-	}
-
-	public function test_get_site_by_slug_with_invalid_network() {
-		$result = get_site_by( 'slug', 'foo', 444 );
-
-		$this->assertNull( $result );
-	}
-
-	public function test_get_site_by_slug_with_empty_string_returns_null() {
-		$result = get_site_by( 'slug', '' );
-
-		$this->assertNull( $result );
-	}
-
-	/**
-	 * @dataProvider data_get_site_by_url
-	 */
-	public function test_get_site_by_url( $url, $expected ) {
-		$result = get_site_by( 'url', $url );
-
-		$this->assertEquals( self::$site_ids[ $expected ], $result->id );
-	}
-
-	public function data_get_site_by_url() {
-		return array(
-			array(
-				'wordpress.org/foo/',
-				'wordpress.org/foo/',
-			),
-			array(
-				'wordpress.org/foo',
-				'wordpress.org/foo/',
-			),
-			array(
-				'foo.wordpress.org/',
-				'foo.wordpress.org/',
-			),
-			array(
-				'foo.wordpress.org',
-				'foo.wordpress.org/',
-			),
-			array(
-				'www.wordpress.net/',
-				'www.wordpress.net/',
-			),
-			array(
-				'www.wordpress.org/',
-				'www.wordpress.org/',
-			),
-		);
-	}
-
-	public function test_get_site_by_url_with_empty_string_returns_null() {
-		$result = get_site_by( 'url', '' );
-
-		$this->assertNull( $result );
-	}
-
-	public function test_get_site_by_url_with_invalid_url() {
-		$result = get_site_by( 'url', 'not a url' );
-
-		$this->assertNull( $result );
-	}
-
-	public function test_get_site_by_domain_subdomain() {
-		if ( ! is_subdomain_install() ) {
-			$this->markTestSkipped( 'This test is only valid in a subdomain configuration.' );
-		}
-
-		$result = get_site_by( 'domain', 'foo.wordpress.org' );
-
-		$this->assertEquals( self::$site_ids['foo.wordpress.org/'], $result->id );
-	}
-
-	public function test_get_site_by_domain_subdirectory() {
-		if ( is_subdomain_install() ) {
-			$this->markTestSkipped( 'This test is only valid in a subdirectory configuration.' );
-		}
-
-		$result = get_site_by( 'domain', 'foo.wordpress.org' );
-
-		$this->assertNull( $result );
-	}
-
-	public function test_get_site_by_domain_with_empty_string_returns_null() {
-		if ( ! is_subdomain_install() ) {
-			$this->markTestSkipped( 'This test is only valid in a subdomain configuration.' );
-		}
-
-		$result = get_site_by( 'domain', '' );
-
-		$this->assertNull( $result );
-	}
-
-	public function test_get_site_by_path_subdomain() {
-		if ( ! is_subdomain_install() ) {
-			$this->markTestSkipped( 'This test is only valid in a subdomain configuration.' );
-		}
-
-		$result = get_site_by( 'path', '/foo/' );
-
-		$this->assertNull( $result );
-	}
-
-	public function test_get_site_by_path_subdirectory() {
-		if ( is_subdomain_install() ) {
-			$this->markTestSkipped( 'This test is only valid in a subdirectory configuration.' );
-		}
-
-		$result = get_site_by( 'path', '/foo/' );
-
-		$this->assertEquals( self::$site_ids['wordpress.org/foo/'], $result->id );
-	}
-
-	public function test_get_site_by_path_with_empty_string_returns_null() {
-		if ( is_subdomain_install() ) {
-			$this->markTestSkipped( 'This test is only valid in a subdirectory configuration.' );
-		}
-
-		$result = get_site_by( 'path', '' );
-
-		$this->assertNull( $result );
-	}
-}
-
-endif;