Yeah, I don't know how many routers support RFC 5549 and it never seemed to become a best practice.

I think the thing is that if you actually need only 24ish bits worth of addresses for something and it doesn't need to be routable, ipv6 just adds a lot of baseline complexity ipv4 doesn't have unless you go out of your way. Ipv6 isn't just an address extension, it's a whole ass architecture unto itself.