So it seems that the logical labels should be “classic, classic/block, and block” or possibly “classic, classic/block, and FSE”.
That would make it easy to understand the capabilities/features of specific themes. I am currently using classic/block themes, on the most recent site not using classic editor at all, but the theme is a (relatively) newer classic theme (Chaplin, 2021) built to be block-friendly but still supporting a forum. I tried setting up the site with a FSE theme and found it still too confusing; also, it seems that FSE and fora don’t mix well.