I was curious about this as perhaps a new approach to recipe-oriented bytecode injection. But the I looked at the project's (long) history and docs status.
It seems that the main contributor has tapered off this project over time, and more specifically hasn't been active since July. (Mumfrey, if you're out there, please tell us what's up)
I would say that the feature set is pretty powerful as is, and it shows in the earlier history (having worked with Mixins since before it was called Mixins). The history reaches further back than the first commit to having been developed for Mumfrey's own modloader (LiteLoader).
As someone else already replied, Mumfrey still practices private development and only pushes new code after his own extensive testing. He has long periods of "public" inactivity because code refactors and other changes can be delicate given the widespread usage of the library within the Minecraft modding ecosystem, and as such, he doesn't want the distraction that in-development can attract from said community.
IIRC, at least a few years ago, Mumfrey had a tendency to develop Mixin in private and only push the commit backlog for releases, leading to periods of time where no activity was publicly visible. (Also IIRC, this is part of the reason why the FabricMC fork exists.)
So, OP, what made you mention this here now?