I don’t know how long ago I slept through this change, I apologize, if this has already been discussed, please poke my nose… The point is that the server settings have disappeared: previously it was possible to regulate the period for saving messages on the server, but now this setting has disappeared. I would really hate to leave my correspondence on someone else’s server and I vote for returning the “delete from the server immediately after delivery to the recipient” setting!
If you are using single-device mode, your messages are already deleted from the server upon fetching. If you are using multi-device mode, the messages can’t be deleted, as then the other clients couldn’t fetch them. Also, chatmail servers have a retention that already deletes messages after a few weeks.