I just noticed that in this thread: How can I select to keep data from a chosen site when I clear Private data, most of the names aren't clickable.
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The question got migrated from StackOverflow and the user doesn't have a linked account on SuperUser. Therefore, it's a ghost account. They can reclaim the post by associating their account with a SuperUser one.
For completeness, there is a second cause of this; if the poster's account has been deleted for whatever reason, then they show in that way. There are various reasons to be deleted: