The game was 2d, and I just used the stuff in SDL_render.h for doing the graphics.

SDL2 has input stuff built in, so that is easy.

I didn't need networking for my game, so I didn't have it, but networking is not particularly difficult to do cross platform (I'm assuming iOS lets you just use the "normal" sockets API?)

I also used SDL2 for the sound stuff, which was mostly playing back prerecorded things, but also a little bit of stuff synthesized on the fly.

SDL does have a platform-agnostic render library for basic stuff, and you can integrate Metal into it pretty easily.

You have an OpenGLES 3.0 C API on iOS but it's deprecated (still working fine though).

But SDL3's has its own 3D API now which (I assume) wraps Metal on iOS.