We stopped doing carrier and rooting/roms/hacks subforums once we went through our massive forums re-organization. Older devices still have these subbies.
This is actually the first public 'request' of this (that I've noticed anyway!) since the re-org was completed (many months ago). If you feel it would be worth it, prove it to me! Some things to keep in mind:
1. Too many subdomains litters the forum index with forum after forum, making the forums home longer than it needs to be.
2. If we title our threads properly in the first place, we can eliminate the need for some subbies.
If you know me I'm completely open to anything, provided we have evidence that it will make things even better around here!
I know many on our mod team were used to the setup you're requesting here (and that's cool)... but until now they have been the only ones to request this. Also, if we were to move back to this sort of structure, it'll leave a lot of thread moving for our mod team to do.
All stuff to keep in mind. If this were to help create more action/thread creation in the HTC One, SG4 forums (as two examples), could be all good. Sound off!