You're right - found https://lists.sr.ht/~sircmpwn/aerc/patches/8864 now aswell, it indeed uses the workers for threading. I'll give it a try, and will also take a look at adding notmuch support :-)
I'd also be very interested in a threaded view. However, I don't think assembling threads should be done client-side.
IMAP seems to support threading, and backends like notmuch are also probably much faster when loading threads or displaying query results in threaded view.