When meta boxes are registered, they can use the `__back_compat_meta_box` and `__block_editor_compatible_meta_box` flags, to show whether this registration just exists for if the classic editor is loaded, and whether this meta box is compatible with the block editor.
When a meta box marks itself as incompatible with the block editor, and `WP_DEBUG` is enabled, a warning will show inside that meta box in the classic editor.
As all core meta boxes have been recreated in the block editor, they can be marked with the `__back_compat_meta_box` flag.
Merges [43779] from the 5.0 branch to trunk.
See #45112.
git-svn-id: https://develop.svn.wordpress.org/trunk@44132 602fd350-edb4-49c9-b593-d223f7449a82
23efeb45c8