Is it really not UB to attempt to modify RO things? Typically (on major implementations, e.g. GNU/Linux or MSVC/Windows) it will crash, and I don't think it is

Is it really not UB to attempt to modify RO things? Typically (on major implementations, e.g. GNU/Linux or MSVC/Windows) it will crash, and I don't think it is allowed to crash on non-UB things?