That's true, of course, but SIMs can be reprogrammed by the carrier on a whim. Plus, there are handover features that command the modem to downgrade the connection from the network side, and who knows if the modem will listen to the SIM's config if the network commands it to do something.

I haven't needed to enter APNs in years, there are standards to provision those by SMS if they're missing and most of them are pre-configured in the phone's OS.

I think limiting this at the modem side will be more effective than reprogramming the SIM card, but the specifications are open enough that you could take a look at a SIM's contents by throwing it in a reader.

You could also look at the code and blobs dealing with eSIMs, as they provide the same features but often come packaged in the form of software.

Check your local laws before you start messing with SIM cards, though, altering certain identifiers can be a crime.