Is it possible to un-join a game without pressing the "decline" button? like you go back to the "initial state" when you haven't joined nor declined

no. that isn't possible.

why would that be necessary? if you join, you want to play. if you don't, you decline. if you're on the fence, declining is a great option, since you're apparently not enough into it in the first place

It's a chat game

interesting. well, sounds like you're going to have to figure out if you care about the chat enough to stick around or leave, because I doubt that's a priority update

even priority things don't get updated. this feature isn't even required.

I'm guessing you wanted the unjoined game to float to the top of your list though? Or were you thinking of clearing your chat somehow as well? :P