On https://www.openstreetmap.org/changeset/175272266
we see there is no way to use "@" syntax on
people with space in their usernames.

Apparently the trick is to use %20 for space.

But alas, there is no GitHub-style @ list to pick from, so it is easy to misspell other peoples names, causing them to not get notified (if indeed they are especially notified, via these "@"s, in the first place, even if not subscribed.)