If I understood correctly, @RMerlin has stopped working on the 382 branch due to unfixable bugs in closed source components.
As for the 384 branch, there's a pre-release test build available (see this thread: https://www.snbforums.com/threads/preview-asuswrt-merlin-384-3-pre-release-test-builds.44060/) also for the RT-AC68U. Keep in mind, this is a first alpha, so you might run into (still to be solved) issues. I've been running it for several days now and even though QoS still has issues and some minor cosmetic things, I'm pleasantly surprised for it's stability, given the fact that is a first alpha.
As for your last question: I can't speak for @RMerlin off course, but I'm confident that as long as ASUS supports this model (and they still do) and @RMerlin keeps up his continious efforts to develop and maintain Asuswrt-Merlin, we probably have nothing to worry about and can keep using our RT-AC68U for quite some time.
Ah I see, ty wi-fi still needs to catch up to 1gig, so I'd hate to see this router expire as it's a perfect compact device and covers the entire house with just one
So ASUS botched 382 with closed sources so we're jumping directly to 384? sounds exciting
Is the AiMesh included in 384? I've read the initial announcement from RMerlin but I was confused on where it will be released with.