Many sports leagues had those waivers and they're null and void under certain conditions , like hiding potential hazards like nfl did with CTE, or if courts find them negligible , for say ohhh I don't letting someone fight 3 times in one night lol
those waivers usually ain't worth the paper they're written on, and mainly for discouraging people from taking legal action.
Of course a fighter could not sue for breaking a leg or some shit like that , which is a clear and present possibility, but for hiding something , or being negligible in some way, pretty sure that waiver is useless.
Though, i would imagine with mma , the fighters are forewarned about head trauma and CTE, brain bleeding and all that shit, IDK. lol.
Long story short is I for one would still go after the league who let me fight 3 times in one night no matter signing a waiver