In reply to Justin Tadlock.

Eventually, "old" themes will become obsolete and incompatible. The big question mark is find out when.

I've been exploring the possibility of coding themes for both old and new (block-based) themes. I already code my themes for the classic and the block editor. So, perhaps that means my original flavour list will now look something like this:

WordPress with the classic editor
WordPress with the core GB
WordPress with the core GB + the plugin version
WordPress with the core GB + Classic editor + Plugin + Block areas