> Does there exist any platform which has multithreading but not atomics?

Yes. Also, almost every platform I know that supports multi threading and atomics doesn’t support atomics between /all/ possible masters. Consider a microcontroller with, say, two Arm cores (multithreaded, atomic-supporting) and a DMA engine.

The boring answer is that standard atomics didn't exist until C++11, so any compiler older than that didn't support them. I think most platforms (certainly the popular desktop/server platforms) had ways to accomplish the same thing, but that was up to the vendor, and it might not've been well documented or stable. Infamously, `volatile` used to be (ab)used for this a lot before we had proper standards. (I think it still has some atomic-ish properties in MSVC?)

Certainly such systems can pretty readily exist. You merely need atomic reads/writes in order to implement locks.

You can't create userspace locks which is a bummer, but the OS has the capability of enforcing locks. That's basically how early locking worked.

The main thing needed to make a correct lock is interrupt protection. Something every OS has.

To go fast, you need atomic operations. It especially becomes important if you are dealing with multiple cores. However, for a single core system atomics aren't needed for the OS to create locks.