Considering the implications of relying on an external company as the registry, I don’t think custom domains are really “vanity” as much as reserving agency to move the code if it becomes necessary. I’m perfectly happy with GitHub, but would rather my modules didn’t break if they implement a policy change at some future date. I also don’t like the implication that “GitHub owns” my repo due to the import path stuff.

That being said, I wonder if this same thing could be achieved with a simple reverse proxy/CDN with a few rewrite rules? Ideally, the only cost to a typical maintainer would be the domain name, and the rest could be hosted on free infrastructure (cloudflare would seem like a reasonable choice).