Right now, mention emails are enabled for newly created users (but not users that existed before Echo was rolled out) by default, but those "new user overrides" are only applied to non-autocreated accounts. This means that, by default, newly created user accounts get mention emails only for mentions on their home wiki, not for mentions on any other wiki.

However, the flip side of this is that if we remove the autocreated check, then if a longstanding user (say, one created before Echo existed, and who does not have mention emails enabled anywhere) visits a wiki they've never visited before, the account that's autocreated for them would have mention emails enabled. If we care about that, we could change this check to look at the registration timestamp of the global account, rather than the autocreated-ness of the local account.