I actually like the idea of what is being proposed and would help differentiate classic to block themes. I won't comment on the FSE type theme--saving that for another day.

I create my themes from Rough Pixels to be compatible with WP Classic, the block editor, as well, as ClassicPress. However, as things keep progressing, I'm starting to move to make new themes that are specific to the editor.

If this proposal goes ahead and becomes a reality, I will be more inclined to make "block-specific" themes; while keeping the classic versions separate.

I will be releasing a new theme soon -- once I get back from my much needed vacation :)