Returning error codes in-band is the reason for a significant percentage of C bugs and security holes when the return value is used without checking. Something like Rust's Result type that forces you to distinguish the two cases is much better design here. And no, you are not working with a whole language ecosystem of "sufficiently disciplined programmers" so that nobody ever forgets to check a return value.

Not to mention that errno is just a very broken design in the times of modern thread and event systems, signals, interrupts and all kinds of other ways to produce race conditions and overwrite the errno value before it is checked.