I'd rather it broke. If someone linked to the old URL, describing it as "the profile page of an international child murderer", and I later inherited that URL by signing up for the service, I wouldn't be too pleased.
The "y" example is confusingly simplistic; the site already has a "can't use duplicate usernames" rule, so 404'ing the original URL isn't 'worse' for anyone than if the original user didn't delete their account.
The "y" example is confusingly simplistic; the site already has a "can't use duplicate usernames" rule, so 404'ing the original URL isn't 'worse' for anyone than if the original user didn't delete their account.