They should notify you, but I’m sure they probably do. Some sites have newsletters they email out that you can opt out of. I think they probably don’t want to bother you every time they update their terms. I assume most of the time they’re doing it because they have to legally, not because they’re trying to target their own users.
I think most of the time, gambling sites are not notifying their users.
On the other hand, it's just a rare case that when we are not even notified, we are still safe on our usual doings.