Ok, so you are actually mixing in email compatibility.
This aspect of message handling is to be dealt by the first chat client that notices the message. It moves the chat message out of the way of classic clients as soon as possible and reliable. It’s about moving chat messages into the chat folder, to avoid further email clients to download it from the INBOX.
EDIT: What is missing is still a solution to the usage-problem 4 (double notifications) at [Wiki] Use-cases, chat rules and configuration options, but this needs to work more generally, i.e. even before deleting messages after reception (before downloading or reading).
There is something you need to explain, if you want “traffic-minimization” to break message delivery to additional chat clients. (Note that multi-client syncing of the past chat histories is something different, that could be made gradually configurable, but would also be disabled completely by a “traffic-minimization” option that deletes messages from the server after they are read.)
Please explain: How many times do you think an unread message would get re-downloaded in your use-case unnecessarily, before it’s removed from the chat folder after it was read? (i.e. re-downloaded without the user actually wanting it, in order to be able to read the new message)