Maybe it’s not mandatory, but I thought it was. I thought it had to do with a goalie several years ago who got hit in the throat with a puck and nearly died.
I am no rules expert. But I think it has to do with the type of helmet worn. Ward’s and others have a fairly decent ‘drop’ on the chin of the mask. Others, like Dominick Hasek wear an off the shelf ‘cage’ helmet that doesn’t have that and the neck is totally unprotected, thus the additional shield is needed.