You don't deprive someone of significant amounts of oxygen by putting your knee on their back or shoulder or even back of the neck. Trust me. I've been in countless matches with people in these positions. I've seen women and younger judoka in these positions. In Judo we don't even apply air chokes because they are so ineffective and take too long. A perfect air choke can still require 1-2 minutes. Blood chokes are the preferred choke in UFC, Judo and other grappling sports. You have to train to apply them and you apply them with your hand or arm not your knee or leg. In BJJ you have a triangle choke with the legs but that still involves using the hand to bring the head into the choke. If these chokes are perfectly applied the person just sits there and is annoyed not passing out.

The only way you're going to kill someone by sitting on them is if they are on drugs, or you have a heart condition, or other preexisting condition that makes you prone to heart damage during any physical confrontation where heart rate will be elevated.

Tony Timpa was also restrained by officer this way but he was restrain for 13 minutes not 8 and he probably only died because the paramedic gave him a sedative after he'd been restrained thus lowering his heart rate even further.

Floyd died because he was an old drug attack with heart conditions and was on drugs during the confrontation. You shouldn't try to resist arrest if you're on massive amounts of drugs or if you have heart condition and especially not both.

Timpa's family should have received a payout though. Timpa called cops to help. He wasn't the one breaking the law.

Every 5 or 10 years we have a rash of deaths in schools because kids learn about the 'choke out' or 'pass out' game. Even these games are really mostly blood chokes applied to the cartiod artery. They are not 'knee on your back oxygen deprivation' chokes. Most of the kids that die from these normally have heart conditions or something else happens like they hit their head.