> On the other hand, signed integer overflow being UB would count for C/C++

Rust defaults to the platform treatment of overflows. So it should only make any difference if the compiler is using it to optimize your code, what will most likely lead to unintended behavior.

Rust's overflow behavior isn't platform-dependent. By default, Rust panics on overflow when compiled in debug mode and wraps on overflow when compiled in release mode, and either behavior can be selected in either mode by a compiler flag. In neither case does Rust consider it UB for arithmetic operations to wrap.

Writing a function with UB for overflows doesn't cause unintended behavior if you're doing it to signal there aren't any overflows. And it's very important because it's needed to do basically any loop rewriting.

On the other hand, writing a function that recovers from overflows in an incorrect/useless way still isn't helpful if there are overflows.