Limit cellular data usage on iOS
It is not possible to have the phone disable your cellular data use when you reach a certain amount of usage, nor is it possible for it to notify you.

My experience is that most networks offer a service to notify you when you reach or just before you reach the limit, and some offer the option to cut off your data when you reach your limit taking you to a page where you can buy more.

It is unfortunate that this is the way it is, but I wouldn't speculate that Apple presumes everyone has unlimited data, but that data limits are a network issue, and that network providers often provide their own solutions to this, if your network doesn't that is unfortunate, but perhaps a consideration for when you choose your next network provider.