Other languages also have non-local qays of influencing compiler behavior, for example attributes in rust (standard) or compiler pragmas in C (non-standard).

When reading working code, it doesn't matter whether the language mode allows variable reassignment. It only matters when you want to change it. And even then, the compiler will yell at you when you do the wrong thing. Testing it out is probably much faster than searching the codebase for a directive. It doesn't seem like a big deal to me.