Hi @Jeff Gohlke great to read your question and hope you enjoy my answer.
I respectfully disagree that it is "common policy".
The discussions linked to indicate there is actually significant debate about whether or not intros and outtros should be retained or edited out.
The balance appears to have tilted in favor of "remove intros", but IMO a significant contributing factor to that was that @Jeff Atwood (a StackExchange developer at the time) was opposed to intros/outtros and able to action his preference by making StackExchange remove common intro statements at the time of posting.
As benevolent dictator, this is/was his prerogative, but the community preference appears to have stopped short of "edit out every intro/outtro that you see". (I think - perhaps there is more aggressive editing on the busier sites).
I personally subscribe to the "accept intro/outtro removal if part of a wider edit to the post, reject if it is the only change" school of thought. My main concern is that new users (who are most likely to include intro/outtro remarks if they are trying to be polite, and politeness is a great thing and should be encouraged) will be put off by such editing.
Thanks and hope you have a great day!
*Yes, I have deliberately included an extended intro/outtro here in a possibly misplaced attempt at humour.
Edit: Furthermore, if it were common policy we wouldn't need to have this discussion :)