I guess it would be a nice feature to leave a conversation if one is not interested anymore. I had to turn off chat notifications because a dicussion between two others where I havent been involved since ages anymore turned annoying.

you can just ask them to make a new chat without you. Otherwise, I just disable my text notifications and leave all the other ones on (however, this is not the best option when in an important diplomatic game).

A mute feature has been requested before.
I once had an annoying dude who resigned but still felt he had the right to comment on strategy, targets and give Intel. A "block spamming idiot" button was much needed.