Good point. I looked it up.
Slap Shot is from 1977. Helmets weren't compulsory in the NHL still 79/80. And that only for new players. Most were wearing them by 78, in the NHL. Slap Shot is about a minor league, so probably a lot fewer wore helmets.
Apparently some were still playing without helmets in 1996/97 !
(Top Gun helpfully had the name of the pilot stenciled on the front of the helmet! Sort of like why knights had a coat of arms so you could tell who was who and not hack off the limb of your buddy.)