Depends on what you mean by "operating systems". NT, Darwin, and Linux kernels have not been replaced, true. But an operating system is not just the kernel. There have been many different changes underneath the hood that are not as evident to non-technical users as UI revamps. I won't speak for Windows, but Linux saw many innovations both in kernel-space and in user-space, for sure. For example, systemd, Wayland, and PipeWire.
Then there's Fuchsia, which is not only a different OS, but a completely new kernel.
Take a look at Microsoft's MakeCode Arcade. Don't know about libre, but it is open source and you can host it yourself, if you want. Although, the officially hosted version is free to use too.
It's like Scratch, but geared specifically towards making video games. What's cool is that you can run the games on a variety of hardware, including handhelds under $50. Or build your own.
I haven't had a chance to teach using Arcade myself, but I can vouch for MakeCode for micro:bit, which is based on the same platform.