The C23 edition of Modern C

How does "Modern" C compare safety-wise to Rust or Zig?

You'd be surprised: Zig has one UB (Undefined Behaviour) that C doesn't have!

In release fast mode, unsigned overflow/underflow is undefined in Zig whereas in C it wraps.

:-)

Of course C has many UBs that Zig doesn't have, so C is far less safe than Zig, especially since you can use ReleaseSafe in Zig..

UB is does not automatically make things unsafe. You can have a compiler that implements safe defaults for most UB, and then it is not unsafe.

By definition UB cannot be safe.