Development is not finished (in fact, code updates are frequently in the repos) it's just that we're rebooting the assets (characters, maps...), ditching the uglier ones with better ones and that, coupled with the low manwork, is why there hasn't been a new version of OA in a long time.
Relevant links:
https://github.com/OpenArena/enginehttps://github.com/OpenArena/gamecodehttps://github.com/OpenArena/UI3Also, most of the people who develop/play OA is also found on the Discord channel, which is why the forums seem empty.
As for new engine features, I'm afraid that falls into NOTTODO (there's a page in the old OA wiki called NOTTODO which specifies what falls outside of OA scope).