Yes, that is a latent issue waiting for some sucker to try and call it with an IPv6 address!
subnetv4_expand() is called directly from a couple of places in the base system and works.
subnet_expand() is not called by anything in the base system.
subnetv6_expand() is only mentioned inside subnet_expand()
and yes, I can't imagine anything that would require and handle subnetv6_expand() of a /64 (or bigger)!
The only small use for this would be in some places where there are pools. e.g. for V4 it is used in vslb.inc where people can have put a virtual server pool by specifying a "subnet" like 10.11.12.8/29 gives a pool of 8 IP addresses. The front-end GUI code limits it to a maximum size pool of 64 (x.x.x.x/26).
I guess the same possibility of having a group of servers with IP addresses in a tiny IPv6 "subnet" would also work.
The reference to a non-existent function was introduced in 2012 by:
https://github.com/pfsense/pfsense/commit/0917cb214b2bbf7f4b374c901c642987fc4ac63b