This thread is about a year old now, but Chris talks a little about “why Zygote is being replaced with Diffractor and Enzyme”. Probably more stuff in that thread that is of interest.
I recently saw Keno say he would put some time into Diffractor again, and seems like a few commits for starting stage 2 landed last week. So it certainly seems alive, though I think it has been slow progress for a while since some compiler stuff was needed for it to continue IIRC.