Should distributors disable IPv4-mapped IPv6?

Yes, this isn't about the wire. OpenBSD's claim is that giving an IPv4 address two distinct representations (one as sockaddr_in and one as sockaddr_in6) can make it easier to introduce security issues.

Now of course, since OpenBSD is the only OS on the planet that insists on this distinction with no way of turning it off, my personal choice is just to ignore OpenBSD, so that I can have unified handling of everything as a sockaddr_in6. Fewer code paths, less complexity, and more security as a result.