> Any signed addition in C is potential UB unless you have a proof that all numbers that will ever be input to the addition won't cause overflow
This has always been the case. Standard C has always operated with the possibility that addition can overflow. The programmer or library writer is responsible to check if the used types are large enough. If you want to be perfectly sure you need to check for overflow. Making this UB has not changed the nature of the issue.
> is made harder because C doesn't define the size of the default integer types
They correctly made this implementation defined. But C now has different byte sized integer types if you want to be sure.
This has always been the case. Standard C has always operated with the possibility that addition can overflow. The programmer or library writer is responsible to check if the used types are large enough. If you want to be perfectly sure you need to check for overflow. Making this UB has not changed the nature of the issue.
> is made harder because C doesn't define the size of the default integer types
They correctly made this implementation defined. But C now has different byte sized integer types if you want to be sure.