Quite a strange limitation in my opinion. I understand that dealing with subnetting/host network rebinding to allow full IPv6 can be a challenge (especially if you're on DHCPv6 or some other manually managed thing instead of normal SLAAC) but surely Microsoft could use whatever NAT solution they use for IPv4 to work on IPv6 as well?
Apparently support for the protocol can already be enabled (https://github.com/nathanchance/WSL2-Linux-Kernel/issues/25). This makes it technically possible to use a WireGuard tunnel for IPv6 support, at least...