The entire field is a “field of play”. Like I said never in all my life have I seen a player from the opposing team waiting in the end zone for a missed field goal. Never. Is it something new?
Nope. It’s nothing new. And you’re right, the entire field is the field of play. The ball never went out of bounds so it was a live ball until the referee blows the play dead. Great heads up play by the returner.
You do see it from time to time in the case of very long field goal attempts such as this one. It’s just good coaching to put a return man back there and since the clock expired during the play, he’s got nothing to lose by running out a missed kick.
http://nfl.si.com/2013/10/27/cordarrelle-patterson-kickoff-return-record-minnesota-vikings/
It’s a very old rule...just not commonly done.