Screenshot of a profile page in Mastodon with the account address highlighted.
https://media.mstdn.social/media_attachments/files/111/346/835/660/821/224/original/713e7c60efd5e5b5.jpg
"I once saw one profile with bio like username@domain1 and @domain2 to refer to her email address but my instance decided that it was referring to the fedi account of same name as the domain. So, it's not foolproof"
The username isn't in the bio, it's at the very top of the profile (and usually underneath the username on all posts). The user cannot edit this after the account has been created.
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