Why the development for D8 and D9 has stopped?

Why this module hasn't been ported to D8/9, is probably because there hasn't been enough need for it.

I think this is very much a double-edged sword. I've lost track of how many times I've discovered a module that seems to be just what I need, except that it is a version or more behind. Opening issues asking the maintainers about the status of the update often go completely ignored. When something comes out in "Late D7", and many don't even know it is there until "Early D8", or even "D9", and people are asking about the updates, and getting no response, this isn't a good thing.

Would it be worth building this as patches against the contrib Forum module → instead of this separate module?

I started work to recreate some of the improvements on the Forum module, see the related issues for details.