Everyone once in a while I see users (mostly new ones, no offense to them) comment asking for clarification on answers or questions from users that are no longer active on Superuser. They haven't "been seen" - according to their profile - for years.

While answering and expanding on old questions is awesome, and I encourage it while helping new users in reviews, it would be helpful for users to see that their comment directed towards a particular user might be in vain.

Of course, another user could come along and help them out, but if one specifically asks a particular user something, it may be a waste of breath if they are otherwise gone.

Related:

I've also noticed on certain questions that the username is greyed out - I'm assuming this means the profile is deleted. Does this happen after a certain amount of inactivity, or do accounts exist for all eternity unless deleted?