There aren't any real ISPs with no IPv4, but there are a few that support IPv6-only "on the wire", where IPv4 is either translated to IPv6 using NAT64 (e.g. T-M

There aren't any real ISPs with no IPv4, but there are a few that support IPv6-only "on the wire", where IPv4 is either translated to IPv6 using NAT64 (e.g. T-Mobile US) or tunneled over IPv6 using Dual Stack Lite (e.g. Unitymedia in Germany). The main advantage to running v6 on your servers (aside from being a good citizen) is that it allows you to distinguish individual customers for abuse detection, who'd otherwise be NAT'd to the same IPv4 address by their ISP. And ISP NATs are going to become more prevalent, because there just aren't enough IPv4 addresses to go around. What IPv6 provides is an escape hatch; a path that avoids the NATs and tunnels.