In NHL preseason games, the rule has always been that a minimum of 10 veteran players have to be dressed on any given game night. Presumably this was partly due to the fact that preseason games cost the same as regular season tickets and nobody wants to watch a bunch of players they don’t recognize, and also to help entice people to actually attend preseason games (if the lure of hockey being back in your city wasn’t a big enough draw, of course).
With the World Cup of Hockey tournament going on right now, the NHL lifted that rule.