"What happened to the mr Floyd was wrong. "
No shit? Good job you pointed that out...
I learned knee on neck as a restraint in martial arts classes before I did BJJ. It's fucking horrible. You just want to tap from the pain, let alone the fact you're also going unconscious very slowly. I can't imagine what it's like on concrete.
It is never taught in any BJJ classes I've been to. I always wondered why. When I was a white belt in my first year and not entirely sure of the dojo etiquette I was rolling with this blue belt who was way better than me - taller too, really long legs, kept getting me with a leg triangle over and over. He really put it on hard too, so it hurt each time. Eventually I was getting fed up and frustrated by this so I managed to get past his guard - he turned on his side, so I did knee on neck. He tapped, but he got so pissed off with me that when we went again he arm barred me and cranked my arm and didn't let go quickly enough when I tapped. It wasn't too bad at the time because I was warm, but once I cooled down I felt that in my elbow for about a year. I've never tried knee on neck since.
He was ok about it afterwards, but he did say that it felt "horrible". Lesson learned.
It's technically allowed in the rules of BJJ comps (I think), but comes under one of those "dick moves" that nobody does in rolling because it's too nasty, and you risk escalating the situation.
Knee on face is another.