Most spammers aren’t competent to provision an IPV6 service correctly, especially one with matched PTR and SPF(TXT) records, and using throwaway v6 addresses makes SPF much harder. It’s also easy to spot, at which point you can tag the sender domain with a bad reputation.

It’s no surprise that Gmail et al are more interested in sender domain reputation, and also the graph of related domains, than they were a few years ago.