My friend gave me his Polish U/F so I could remove his slant brake and replace it with another. I cannot get the thing to twist at all. It is completely stuck on there. Rifle hasn't been cleaned in thousands of rounds.
All non corrosive ammo and there doesn't seem to be any rust anywhere else on/in the rifle.

Any advice to get this thing off??

Kroil and make sure that you are turning it the right direction.

Is there any chance that it is pinned or welded?
It was bought "new" at a shop in oregon.
I know about the LH threads.
I have an arsenal with RH threads and the brake is loose and just twists off. All AK's i've ever handled have loos brakes.

I assume that the presence of a retention pin above the slant brake means that it is not welded or pinned or else there would be no purpose for it

Heat. Lots and lots of heat. I tried my gas stove and no bueno. Ended up using mapp gas and a pair of vice grips. Came loose pretty easy after the heating. Careful not to torch your barrel much when you do this. I wrapped a wet rag on my barrel near the brake to keep the heat off my barrel. Dont want a barrel turnkng cherry!!

vibrate the snot out of it with an air hammer and nylon tip.  Will turn any rust to dust.