Deleted account and posts still affect how thread are displayed in the thread list.
For example the [Resolved] Site search thread in the feedback forum shows near the top.
I'm not sure what problem you're seeing; that forum thread is near the top because of *****'s reply about a month ago. Also, none of the users in the thread appear to be deleted?
Yep, there's a bug 😅. Thanks for calling my attention to it.