Alright, I have a fix for this:
https://github.com/discourse/discourse/commit/4395e4d165b45a40cc9ba38c1369f6934781c2c2
Because we allow the other flag types on deleted posts I think we should still allow you to send a pm to a user on a deleted post. The downside however is that if you delete the post before you flag it when you send the PM to the user it won’t contain the quoted post you deleted and it will just be a link to the deleted post that the user won’t be able to see anymore. Which probably isn’t a big deal, but wanted to bring it up.
Flag first:
Delete first, then flag and send message:
