because they're just too dangerous to the public or for public consumption? I assume they'd be slight modifications of the more dangerous moves that would be the ones actually taught.
There is a list of illegal moves in MMA. People do not teach them because they are illegal. For example, the UFC rules list:
Butting with the head
Eye gouging of any kind
Biting or spitting at an opponent
Fish hooking (act of inserting a finger or fingers or one or both hands into the mouth or nostrils or a person, pulling away from the centerline of the body)
Spiking an opponent to the canvas on his head or neck
Strikes to the spine or the back of the head.
Throat strikes of any kind, and/or grabbing the trachea
Fingers outstretched toward an opponent’s face/eyes
Downward pointing elbow strike (’12 to ‘6 strike)
Groin attacks of any kind
Kneeing and/or kicking the head of a grounded opponent
Stomping a grounded opponent
Holding opponent’s gloves or shorts
Holding or grabbing the fence or ropes with fingers or toes
Small joint manipulation
Many of these do not have modifications to make them less dangerous; there are no slight modifications of small joint manipulation, stomping grounded opponents, groin attacks, or strikes to the back of the head.