I have a thermos like this and keep it completely clean even tho I make coffee in it daily. The key is to scrub it every day. But I’ve cleaned my sisters after she let hers become far more stained than this. Just needs more scrubbing. If your hand doesn’t fit inside, shove a rough-scrubber sponge or even steel wool down in there with the handle of a scrub brush. Will take some work but it will come completely off if you’re persistent.