Prevent a notice from appearing in the Compatibility Tool meta box

[kienstra]

It looks like this notice doesn't appear with WordPress 5.0-beta3 and Gutenberg 4.2.0.

But it appeared on the single URL validation page, with earlier versions of both. I can't reproduce this (even with WP_DEBUG set to true):

Still, it might be good to ensure that in future versions of Core and Gutenberg, that notice doesn't appear.

This might not be the right flag. But this sets __back_compat_meta_box to true, to indicate that:

...this meta box should only be loaded in the classic editor interface, and the block editor should not display it. (source)

That document also notes:

If you register your meta box with the __back_compat_meta_box set to true, then this warning will never be displayed with your meta box, now, or in future WordPress releases.

If this notice appeared in WP 5.0-beta3 and Gutenberg 4.2.0, maybe another flag would be better.

This is based on this Make post that @westonruter mentioned:
See https://make.wordpress.org/core/2018/11/07/meta-box-compatibility-flags/

[kienstra]

Request For Review

Hi @westonruter,
Could you please review this when you have a chance?

This uses the __back_compat_meta_box flag, which is a little different from what we talked about.

I can't reproduce this notice appearing, but it might be good to pass some flag to prevent this notice in future versions of Core or Gutenberg.