Not usually they aren't. They can be made to be, but it requires extra effort and tradeoffs. Hence why there is a lot of work put into reproducible builds — something you would get for free if compilers were actually always deterministic.
Unless you are taking a wider view and recognizing that, fundamentally, nothing running on a computer can be nondeterministic, which is definitely true.
Not usually they aren't. They can be made to be, but it requires extra effort and tradeoffs. Hence why there is a lot of work put into reproducible builds — something you would get for free if compilers were actually always deterministic.
Unless you are taking a wider view and recognizing that, fundamentally, nothing running on a computer can be nondeterministic, which is definitely true.