Most police departments have outlawed such kinds of restraints. Regardless, using a neck or head to restrain somebody involved in a non violent crime should be prohibited. It’s just not necessary.
My point is, until we see all the evidence. All the video, witnesses etc., we won't know everything that happened. What led to Floyd being on the ground. Did he resist arrest? Did a cop throw him to the ground because the cop was having a bad day? We don't know.
That said, knee to the neck is a terrible way for a cop to restrain someone.