Among the Changes the biggest one is that the NHL will now require every player to wear a helmet in warmups. According to Scoutingtherefs, This is rule 9.6, which last season required players that entered the league after the 2019-20 season to wear a helmet. With the increased focus on concussions and player safety, this seems like an important next step.
Whenever I think of players not wearing helmets, I think of the Taylor Hall incident that led to him getting 30 stitches. Hall was lucky as his teammate, Corey Potter, nearly stepped on his eye trying to jump out of the way of Hall who had fell.