It's been many years since C compilers started making pathological-but-technically-justifiable optimizations that work against the programmer. The problem is the vast sea of "undefined behavior" — if you are not a fully qualified language lawyer versed in every nook and cranny of the C standard, prepare to be surprised.

Many of us who don't like working under such conditions have just moved on to other languages.

Because it is a dead store. Removing dead stores does not sound ridiculous to me and neither is it to anybody using an optimizing compiler in the last decades.

Tree shaking is pretty standard. Optimising out the write sounds fine to me - with the exception of a volatile pointer. That, there, is a mistake.