Once upon a time (maybe they still do) there were shields made that went between the safety and the slide to protect shooters' thumbs if they kept their thumbs on the safety.
I have found myself shooting thumb high a few times, but have never been bitten. IMO, the practice comes from the gun games where stages required a lot of safety manipulation. The downside is that your grip on the pistol is somewhat weakened, allowing it to move in your hand.
The only case of slide bite I've seen was when a chap used a revolver hold, with the support thumb across the back of the gun hand. The slide got him good!