To avoid clutter and tons of stale branches, no other branch than master should be allowed in the main portage tree repo. Project branches have to be in repos for users or teams. I don't know your repo setup, but using gitolite provides easy measures for that :) Reproducible: Always
See discussion on gentoo-dev. https://archives.gentoo.org/gentoo-dev/message/1ea70a56409b70a2d79b2d70096f8907
The main problem I see is that even if branches are merged (e.g. project is finished) or deleted on the gentoo.git repo, they will remain in _all_ repos that once fetched/pulled from the main repo. This _will_ lead to clutter and should not be done that way. The only clean solution is having separate team/project/dev repos.
Can you please discuss this on the list, not here? Reopen if a decision to disable was made.