I first heard about this change when I was following all those spoilers getting leaked, and yeah it's a pretty interesting niche to set Poison-types apart, giving the move a No Guard-like effect when used by a Poison-type.
In fact, it's actually shown in action in this Marriland Pokemon X video, at 6:48 he commands his Sylveon to use Dig and while in its underground phase the opponent's Garbodor uses Toxic on it and it successfully affects Sylveon:
In fact, it's actually shown in action in this Marriland Pokemon X video, at 6:48 he commands his Sylveon to use Dig and while in its underground phase the opponent's Garbodor uses Toxic on it and it successfully affects Sylveon:
Spoiler: