You can always mmap a memory range, place a struct there, then mprotect it. And on the language side you cannot overwrite readonly structs observed by readonly refs (aside from unsafe, which will trigger segfault in this case).

There are ways to do it. What matters is the user ergonomics, otherwise, by this logic, most higher-level languages would have even less of a claim to immutability, and yet somehow it's not an issue?

If there are exotic requirements - there are exotic tools for it. FWIW static readonlys are blocked from modification even through reflection and modifying their memory with unsafe is huge UB.