In reply to paolo.

Sorry but that's not a solution. Having to install and depend on a plugin to keep a "legacy" system working. Sooner or later that will break something. And it will mean you can't use new features without breaking everything.

Meta boxes are such a core part of Wordpress and are being used by developers and agencies for tons of things. It's one of the reasons all those developers and agencies chose Wordpress as a CMS in the first place and not something else. It's one of the reasons not using something like Joomla is that with each version (1.* to 2.% etc) many things break and you have to do a full "migration" to the new version.

I really don't understand why Gutenberg cannot just be used for the content editor, while leaving meta boxes for meta content (!= content) alone.