I have seen this icon a few times, and I just assumed it was a user that no longer had an account.
But then I saw this question which was asked only 10 minutes before I asked this one. What's the deal?
That's exactly what it means.
Sometimes users are deleted by mods for specific reasons. For example, unregistered accounts created purely to post nothing but spam, or overwhelming and obvious trolling. Especially when the usernames are nothing but profanity, we often just delete the 'account'.
In other cases, such as this one, it looks like the account was deleted by the user immediately after it was created. It happens. As for their reasons?
Another case in which this happens is when the question is migrated from a different site. In that case, the original asker/answerers might not have accounts on the new site.