Email masking has become easier to use, and many people use `+addressing` to uniquely tie their email to the service for spam prevention / tracking, which would make stuffing harder.
In these cases, email would be much more unique and a better protection against stuffing. HOWEVER, it’s not obvious how Email verification protocol would work for these types of things.
There are lots of options for running your own Mastodon/ ActivityPub implementation, etc—it’s not easy, though. This is purely a technical problem.
Of course, I am not really talking about Chirp clones here, but this is a pretty good analogue for comparison.
… which means it’s a problem.