Always use master for everything. Of course you can ask someone to help you port PRs to other branches, assuming you want them there - you have to fight for them in shiproom after all. The key point is that each PR has to be first in master before it can be considered to backporting to servicing branches. Normal internal bar for servicing .NET Core applies (even for community contributions to release branches).
Does it help?
why is it closed? If it is something we would take, we should keep it open and set milestone to Future + up-for-grabs (if you have guidance what to do). If you are sure you will never ever do it, then closing is fine. If it is just too far out, I'd recommend to keep it opened + milestone:Future (aka no commitment).