Limit cellular data usage on iOS I have a cellular data contract which offers me a very small amount of data every month. Going over this data limit causes me to automatically be charged for an extra gigabyte, which I want to avoid. 

On Android phones it is possible to set a data cap which prevents you from using a certain amount of data. So, for example, you can set the limit at 100 MB and your cellular data will turn off once you reach that limit. 

I have noticed a significant amount of “system services” data usage on my iPhone (which Apple does not allow you to control or disable all), which is why I am afraid I will inadvertently shoot over the data limit by accident. 

I am aware that there are third party applications which have widgets showing you your data usage, but this is not what I’m looking for. 

One possibility is getting notifications when you’ve reached some limit, such as 100 MB. While not ideal, this helps in controlling data usage. However, my experience with the app I tried, My Data Manager, was quite lousy: it alerted me of my limit at the end of the day, long after I had, without realizing, spent 1 GB  of LTE data.

Another possibility is using a burner Android phone, putting the SIM card in it, creating a personal hotspot and using that with my iPhone. The Android phone would have the data limit instead of the iPhone. However, this is obviously a silly idea in practice. 

So does iOS have a similar feature, either on vanilla iOS, a third-party app (e.g. through an automatically VPN), or some other app which makes monitoring this data passively easier (like the ones mentioned above)? Or is it the assumption by Apple that all iPhone users have unlimited data plans?
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.