Found this new(er) plugin on wp.org from Arrow Design, in Dublin. Free on repo and extremely useful for WP more than for CP, but thought I’d broadcast it here since it helps when still using WP to test-run CP ready plugins.
You can turn on or off individual Blocks usage for posts, pages, widgets or all. This makes it so you can use/enable Classic Editor for posts that you make a custom metabox for; as an example but then leave the Blocks enabled for pages so you can use Blocks for pages (but wont see classic custom meta on pages… for example).