Can Someone Please Explain To Me What A "Defenseless Receiver" Is?

I thought that that was a critical play today when Yeast laid out the Seahawk RB. Wasn't that on 3rd down?

Anyways, though it was a brutal hit for sure, replays showed that Yeast didn't lead with the crown of his helmet.

I mean...if dude had caught the ball, would he have still been considered "defenseless? "

I just get the feeling that defenders can no longer pop anyone unless it occurs in a conference championship or Super Bowl (see Nick Scott on Deebo).