From what I've read, he primary concern with VPNs that do not support IPv6 is leakage. If a user’s device tries to access an IPv6 resource while connected to a VPN that only routes IPv4 traffic, the IPv6 packets can escape the VPN tunnel. This exposes the user's real IP address to external servers, undermining the privacy that the VPN is supposed to provide. Some servers have moved to strictly IPv6. Some servers only accept IPv4.

Some of you networking gods set me straight.