Some answers include the asker's username, particularly on questions asking about how to write an email. If the account gets deleted later, what should we do with the username in the answers?
If the user deleted the account to dissociate themselves from their posts (especially when they used their real name), should we respect their privacy and edit out their username from the answer?
Note that we probably don't know why they deleted their account, so should we just flag it to mods and let them deal with it, or even just ignore it and move on?
While it might be possible to search for all references to an obsolete username, and edit them out one by one, I think we shouldn't do it because it is tedious and to avoid drawing unnecessary attention to their username.