do they disallow it or just not provide active support? Active support requires paying for employees to keep it working. Ignoring it and having volunteers do it requires nothing.
I think the comment up one was about Linux on the iPad, which is mostly impossible. Well, iirc there are some projects to get, like, Alpine Linux running inside iOS, but it is emulated or something, and pretty slow, no gui, quite limited, etc etc.
And that's for macOS. For any other platgorm they actively prohibit any third party operating systems.