Kernels have been developed in safer languages already before C became widespread outside Bell Labs, it is a myth that C is even required for that, other than historical baggage.
Which is extra visible when one looks at original UNIX sources and its many short assembly bits in separate files to handle bits of direct hw manipulation.
What do you mean by this? Like writing to a specific integer address?
*((volatile unsigned *)(0x20001000)) = 0x12345678;
Please cite chapter and verse of the C standard which defines this behavior. Any edition.
Edit: Looks like the result in this case is implementation defined: https://stackoverflow.com/a/24212940
It is, however, not the only way to write to memmapped registers.
The original C way of doing so was to use an assembly function wrapping the actual act of reading/writing (honestly, better than doing the above, as it helps making it very explicit how the write will happen as well as abstracting any details like needing to add a barrier or whatsoever), the other way was to specify the symbol with address of the memmapped register in assembly, and link resulting object with C code.
A C implementation is, AFAIK, free to refuse the literal addresses used as pointers and pass as ISO C.
At least they don't without LTO.
What I mean is, most compilers won't generate a function call or an additional indirect memory access for *(111) = 222;, unlike your spec-compliant alternatives.
This is, IMO, the problem. Sometimes we just want a more ergonomic way than inline assembly in a hot loop (and also we'd like to to pretend its portable). The spec does not have an answer to this.
So, unless all C compilers in the world actively sabotage this usage (like how Go did to map traversal orders), your spec saying this is "implementation defined" have no impact at all. All C compilers are bound to implement the same intuitive but completely-not-enforced-by-spec behavior. Because if they don't, people just use something else.