On an ordinary public website, the user's profile is always publicly visible to all visitors (such as Stack Overflow), where they can limit certain pieces of information via privacy settings or just removing the information. Now the user has decided to delete their account (in my case deactivate) so that their account doesn't technically "exist" anymore. The way my system is set up, when their account is deactivated, their username for any content connected to them just becomes "Anonymous User" as if it were a guest that posted.

I feel like this could cause some confusion for other users. I'm also concerned about what kind of error to display when someone attempts to view their profile page. My gut tells me to just display a standard 404 page to hide the fact that they ever existed, but then you also have to consider that, since usernames must be unique, anyone can go to the register page and type in the username to see if it really exists or not.

I have a similar problem with another website, which gives users the ability to hide their profiles from the public and only allow registered users to view it. Again it's with the dilemma of what kind of error message to display when an unregistered users attempts to view their profile with invalid permissions.

So, would it be acceptable to display basic errors such as "user has been deactivated" or "you must be logged in to view this profile" in order to give other visitors some idea of why the page can't be displayed, or should I attempt to cover the user's privacy a little and just display a standard 404 without indicating in any way that the user might exist? Are there any other issues that I'm not realizing about either route?

To go back to the beginning, should I even bother changing the user's name to "Anonymous User" when their account is deactivated? Would it be acceptable to just display a non-linked version of their username in place of the normal linked display name?