My 2 cents is that this is a mistake / missed opportunity. For the rare cases where a programmer actually wants their integers to wrap (this is like 0.01% of integer variables? Hashing is the primary use case), Mojo should offer a Bin64/Bit64 data type for “opaque binary data”, i.e. the kind of data that you want to shift >> around and have 2’s complement wrapping.
When an “everyday integer quantity” overflows this is almost always a programming error, and it would be very helpful for some of these errors to be detected at runtime, at least in debug builds. This doesn’t prevent you from promising that in release builds, the value will wrap instead of being caught. But this would purely be for performance reasons; this wouldn’t make integer overflow valid/correct.