At least in recent C++ standards, integers are defined as two’s complement. As a practical matter what hardware like that may still exist doesn’t have a modern C++ compiler, rendering it a moot point.

UB in C is often found where different real hardware architectures had incompatible behavior. Rather than biasing the language for or against different architectures they left it to the compiler to figure out how to optimize for the cases where instruction behavior diverge. This is still true on current architectures e.g. shift overflow behavior which is why shift overflow is UB.