Question on an NFL rule

I’ve thought about this a few times watching kickoffs. Ball is kicked deep into the endzone. The receiver when he puts his foot back to kneel down steps out of bounds.

Safety or touchback?

That’s a touchback

Stepping out of bounds there is the same as kneeling

Correct, only a Safety if the receiver establishes position in the field of play with possession of the ball and then retreats into the end zone.

Touchback. He was never in the field of play.

It’s also a safety if a ball carrier from scrimmage steps out of the back of the end zone, but that doesn’t apply to kickoffs.

It’s also a touchback if a defender intercepts a pass in the end zone and runs out of bounds (or falls to the ground in the end zone) before crossing the goal line.

True, and also if the interception begins out of the end zone but ball control is only established in the end zone whereupon the defender goes down or out of bounds, it’s still a touchback as control was never established in the field of play.