"was never guaranteed to work by the C standard"

It was, on x86, before of compilers emitting opcodes not supporting unaligned memory accesses, because x86 CPUs guaranteed safe operation on unaligned memory accesses.

Don't get me wrong, of course it is a good practice writing code not relying on CPU-specific characteristics. The problem is that there is a ton of software for x86 that becomes not reliable when recompiled with -O3 on x86-64 (with hard to catch situations, e.g. passing tests, but with random crashes depending on the data being handled, etc.).