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.
Built from https://develop.svn.wordpress.org/trunk@44132
git-svn-id: http://core.svn.wordpress.org/trunk@43962 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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