Not allowed to put things in seat pocket?

Last week I flew LAS-DEN. This is not about moving seats, although they did mention that nobody could move too! A new one - they announced multiple times that we could not put anything in the seat back pocket because of safety reasons while taking off or landing. They walked through and checked, too.

I always have my kindle, snacks, and phone out of my bag before taking off and put them in the pocket so they’re accessible. I keep my backpack under the seat but I’m short and prefer window seats so have a really hard time getting into my bag once someone is next to me and we are moving.

I have never heard that before - is this a new policy? Or just something they were doing for that flight specifically?