They need to ban the oblique kick. You can get instant ligament damage from it. At least with submission attempts, the opposing fighter knows a sub could be coming and can tap before it happens. Whereas with this kick, you don't have any warning and then when it lands properly, can instantly turn your knee into mush. This is one of the more severe examples of it, but we've seen fighters get their leg torn up from this. Look at Whittaker vs Yoel and how it fucked Whittaker's knee up from the start.
IF we gonna continue to let this move exist, then we need to allow head butts, knees to downed opponents, kicks to the head, groin strikes, etc. I mean, its a fight right, so everything should be legal? Or should we consider this a sport and have some rules in place to help protect the longevity of its fighters? Obviously brain damage is worse than ligament damage, but the latter will force you out of MMA quicker than the brain damage will (usually shows up later in life).