The affected platforms lack an OS (e.g., bootloaders) and/or an MMU/MPU (e.g., microprocessors like AVR)
BLAS, gemv, GEMM, SGEMM libraries are from 1979, 1984, 1989. You may have seen these words scroll by when compiling modern 2025 CUDA :)
C has no backwards compatibility guarantee, and it never has. Try compiling K&R C with gcc's defaults, and see what happens.
You can build your legacy code with legacy compiler flags. Why do you care about the ability to build under the modern standards?
In AVR or other MPU-less architecture you can literally modify the string literal memory without triggering a crash.
Why? Because there is no memory protection ("rodata") at all.
And such microprocessors are still in use today, so it's a bit too far fetched to say "really old code."
It's UB, sure, but how many embedded programmers actually care? The OP's proposal is trying to change the type system so that this UB becomes much less likely to trigger in practice.