I would assume that the ‘fucking annoying’ + thing is how discourse
deals with threading (and branches).
That’s what the References and In-Reply-To headers are for. If that’s the purpose the tag in the Reply-To header is serving then it is indeed completely unnecessary.
Allowing people who use Elm or Pine to completely clear everything out
I don’t understand this. What does “clear everything out of an email” mean, and why would you do it?
This email was created by starting a new email to the address.
If I create a new email instead of replying I intend to create a new thread.
Note it doesn’t even have the same subject line.
That’s not visible in the emails I received, it’s still “Re: [HacMan] Mailing list dups”.
As Alan said, sometimes you /want/ to change the subject within a thread, but keep the threading intact. What you say is naturally in response to another post but the course of the conversation has changed.