In reply to Michael.

As far as I know the initial plan was to display legacy metaboxes below the Gutenberg editor in a “Extended Settings” section.

You are absolutely right, this is the current plan and discussion being said in the issue focusing on metaboxes. Thank you for sharing that and glad to see you are following the current progress.

There's a lot of content in this post, so forgive me for taking a little time to clear up a little confusion. What was actually said in the comment was there would be a plugin that "gives you the legacy/old edit page". It wasn't said that you'd need this to simply use metaboxes. The community is having a discussion on those and working on a solution - which is really hopeful as seen by recent work.