user:<id> option to the search string allows you to search for posts written by specified user. That also includes the wiki posts, which are considered community owned (almost everyone could modify them). But I was surprised that searching by user doesn't work for deleted users, despite of the fact that user id is still visible on the user card:
Could the searching for posts made by currently deleted user be possible? Is there a reason to don't allow it?
One possible usecase is when the user with a lot of useful posts decides to delete your account, and you will be able to search for they posts though.