I don't really see any rule change here? I officiated HS for 20 years and as long as the offense is set for a second. A single backfield player (or slot receiver) can go in motion. I receiver on the line that resets into the backfield must pause for a second before he can go in motion! I think these are more rules Emphasis because teams are not setting the proper full second prior to going in motion. Also, players in motion going are moving toward the LOS at the snap (that is an easy illegal motion call). You MUST be in motion parallel to the LOS or backwards...This is not the arena league!!
In the video like it or not...Tyreek is moving toward the line at the snap which is an easy illegal motion call. The Rams were doing the exact same in this video! The SF player, I view as a legal play because he was moving parallel to behind the line at the snap. Who cares if the get a head start...no one cares... as long as they are NOT heading towards the LOS at the snap! I am a huge Dolphin fan, but rules are rules. Otherwise, the NFL ends up like the NBA where the travel call no longer exists!! Take as many steps as they want!