Is it safe for a baby to sleep with arms out in a sleep sack?

Yes, once baby has outgrown the swaddle stage. Reddit threads in r/sleeptrain (tagged high-trust in our data) note that arms-out sleep sacks are the standard recommendation after rolling begins, and brands like HALO and Burt's Bees Beekeeper are sized to keep the neck opening secure so the sack can't ride up over the face.