Interesting. I assumed all VPNs switched to IPv6 by now, making detection much harder.

Much of the internet still does not support IPv6, so most providers will give you an IPv4 address. In fact only a few providers even support IPv6 at all.

Even with IPv6 it's not a huge problem. With a few samples we can know that a provider is operating in a given /64 or /48 or even /32 space, and can assign a confidence level that the range is used for VPNs.

IPv6 isn't magically unrouteable, it just routes much larger blocks of "end IP addresses."

You just track and block /24 or /16 as necessary.

Many websites including Soundcloud are still only accessible through IPv4, so this is moot, even if VPNs support IPv6 it's enough to block their V4 exit nodes for Soundcloud.