Sometimes you're writing code where it really, really matters and you're more than willing to spend the extra cycles for every add/mul/etc. Having these new types as a portable idiom would help.
N2466 2020/02/09 Svoboda, Towards Integer Safety
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2466.pdf
The committee asked the proposers for further work on this effort.
Integer types that saturate are an interesting idea. Because signed integer overflow is undefined behavior, implementations are not prohibited from implementing saturation or trapping on overflow.
The entire notion that "since this is undefined behavior it does not exist" is the biggest fallacy in modern compilers.
You're exactly right that this is why there is a distinction between conforming and strictly conforming code.
Implementations define undefined behavior all the time and users rely on it. For instance, POSIX defines that you can convert an object pointer into a function pointer (for dlsym to work), or implementations often rely on offsets from a null pointer for their 'offsetof' macro implementation.