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 do not notify their users.
Usually it only happens to small casinos, it won't happen to big casinos, which of course protect their reputation better, so they have to be transparent in changing the TOS, whatever that is, it's important for users to know, because if it's done without the user knowing, it's certain that the casino has the intention to cheat users, then from that's probably something that we often only find in small casinos.
No big casino so far I've seen Closed silent on TOS changes because I play in some big casinos almost on average they update TOS always announce it via email to users or through their site for users to read the rules and terms that apply, therefore why it is highly recommended to play in casinos that have a high reputation as opposed to new and small casinos.