I just saw a couple of new patch sets posted by MediaTek developers to this 21.02 feed, as mentioned by someone on the R4 big thread on BPI forum.
So I would speculate the process works like this:
For the time being all development on R4 done by MediaTek will be on version 21.02. They probably will rebase to a new version in 3 to 5 yrs if FiLogic 880 sells really well e.g. like MT7621
OpenWrt developers will pick up patches from the feed, and port them to latest OpenWrt version. While doing so, upstream most of all changes to the mainline Linux.
So if someone is interested in testing, possessing development skills, it doesn't hurt you to pick up some new changes from the feed and test. Then perhaps submit a pull request to OpenWrt git (?). Sort of becoming like a OpenWrt developer too.
--
BTW, I also saw on the BPI forum someone seems to get the PWM fan working