If it doesn't support IPv6 it doesn't work.

Not only that, but not everyone will even have any other choices. The last apartment I was in literally only had one ISP option; I literally would check every six months or so with other ISPs that were in the area because of the fairly frequent outages, and every time they all said that they couldn't offer me service at my address. (This didn't stop them from filling my mailbox with spam all the time though of course). This was in New York (the city), so it's not like there weren't half a dozen other ISPs operating within a few blocks of me.

I can't take seriously the claim that someone would literally refuse to move into an apartment purely on the basis of not having IPv6 support. Bad internet in general? Sure, that's plausible; I work from home, and like I said, the outages were annoying, and if there were no decent speed options my (now) wife and I might have ruled it out? But literally just the lack of IPv6? That's an absurd reason to pick another place to live entirely.