I’m familiar with proprietary APIs where a third party decides what data and functionality is available to developers communicating with their server.
However, secp256k1 is an open source cryptographic library which is entirely accessible/callable and can be forked. What is the secp256k1 API in this case and in what ways is it restricted to limit functionality?
In this GitHub comment it is suggested that people back in 2015 were "attempting to use the library calling into random internal functions. That should be harder to do now". Why is this discouraged and why is it harder to do now?