While UUID4 can be good for UserID. Sometimes, to avoid having a second index in the DB, ULID is another good option; As not only it supports some of the features of UUID4 of uniqueness, it is also sortable.
TIL about ULID[0]; It looks interesting, but the spec was last touched in 2019 and I haven't really heard of it before... is it actively used?
Also curious because I don't actually know: If a format spec is GPL, does that encumber implementations of said spec?