Is syscall a public API on iOS? In the end, you have to call that to get anything on the screen?
Looking at unistd.h, it seems marked as
__OS_AVAILABILITY_MSG(ios,deprecated=10.0,"syscall(2) is unsupported; "
"please switch to a supported interface. For SYS_kdebug_trace use kdebug_signpost().")
#ifdef __APPLE_API_PRIVATE
Not at all, it is a Linux thing to keep applications doing syscalls, like back in MS-DOS interrupt days.
All other modern OSes give zero guarantees about syscalls.
Indeed, you have to call UI Kit, that is the public API for userspace applications.
Even if via OpenGL ES or Metal, you need a drawing context and a Window to render it.