I feel like there's an easy win to keep up with the fragmentation of discussions without waiting for some implementation of this feature request.

All a frontend needs to do is group all posts with the same URL together and display all their comments in the as one unified comment section. If you reply to the OP, you can either choose which community the comment goes to, and maybe set a default as well.

This functionality should be an extra switch for the frontend, so that the user can disable it and see individual posts.

This also nicely avoids not knowing how to deal with moderation, as each community moderator still maintains control.

Comments from blocked communities would not appear ofc.

This would both prevent seeing the same post multiple times on your feed, but also drive view to smaller communities where comment in their sections are ignored.