Are there really ISPs that don't support IPv6? I've had IPv6 from various ISPs since around 2010, and even my phone gets an IPv6 address from the cellular network.

Yes and it's ANNOYING. In Switzerland there is literally not one cellular network that issues IPv6 addresses. Also my workplace network (a school using some sort of Microslop solution) doesn't issue IPv6es.

I have a IPv6-only VPN with some personal services. Theoretically, the data can be transported via IPv4, but Android doesn't even query AAAA records if it doesn't have a route for [::]/0. So when I'm not home, I can't reach my VPN servers, because there is supposedly no address.

(I fix it by routing all IPv6 traffic through my VPN. Just routing connectivitycheck may suffice though).