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As we expand on SSO getting more "foreign" software integrated with MusicBrainz/MetaBrainz authentication, we will run into issues that the external software may not deal gracefully with allowing pretty much all characters. E.g., https://musicbrainz.org/user/Fr@nklyn gets turned into https://community.metabrainz.org/u/Fr with Discourse, which will cause conflicts when "Fr@nk" or "Fr@ppuchino" etc. registers. (There should possibly also be some better mapping in Discourse to help with this (e.g., that their SSO implementation changes "@" to "a"), but that doesn't mean it isn't something we may encounter over and over again (e.g., what limitations does MediaWiki have if we want to do SSO on the wiki? Jira?).)
I will not suggest changing already existing accounts, but it may be something we want to look into soon whether we want to limit available characters for names for new accounts.