I say this with great care as I do not want to launch a flamewar.
If you do not consider Linux with namespaces an OS (because of fragmented userland): Would you then consider FreeBSD with jails or Solaris with zones for fully fledged?
If you still consider those flawed (maybe because thet do not force you into jails/zones) should we at least no consider OS/390 or z/OS as proper operating systems to that/your (not meant inflamatory!) standard?
Yes. Though you do not mention them directly DOS and Windows has ruled the world for years and they opened the door for the nasties. But they were not all there was - only the popular/easy choice. Everything is a trade off.
Isolation mechanisms is not what makes an OS. It's the stable ABI that application developers can depend on and which provides a way to use shared resources: disk, CPU, RAM, GPU, network, screen space, push notifications, GUI integrations, your favorite LLM integration, so on, so forth... Yes, it might have an imperfect security model, but nothing's perfect under the sun.
Raw Linux without userspace could be considered an OS, but it has the ABI only in form of syscalls and the minimal standard FS. That's barely enough for anything other than, say, a statically linked Go binary, which is why it's seldom used by app developers as a target.
To most of your examples I say – yes, that's an OS, and jails or zones have nothing to do with it. Although I'm not familiar with them other than FreeBSD, so I'm relying on your short description and your implied criteria for selecting these examples.
If you do not consider Linux with namespaces an OS (because of fragmented userland): Would you then consider FreeBSD with jails or Solaris with zones for fully fledged?
If you still consider those flawed (maybe because thet do not force you into jails/zones) should we at least no consider OS/390 or z/OS as proper operating systems to that/your (not meant inflamatory!) standard?
Yes. Though you do not mention them directly DOS and Windows has ruled the world for years and they opened the door for the nasties. But they were not all there was - only the popular/easy choice. Everything is a trade off.