You can legislate it to the point that it’s only possible to spread or hand to hand, and doing so is illegal. Hell, they could go a step further and make just having a copy of it within your house illegal, even if it wasn’t installable.

In any real terms, that means the only way to keep a foss os is to build it entirely yourself. Not build it from source, but code the source from the ground up. It would take time to happen, but when enough time passes, both only do the skills necessary to do that fade, but hardware changes being closed would add another layer of difficulty. Look at the pain in the ass that android rom developers have trying to get AOSP going on a given device.

I get it, I get the dream of a dedicated and determined underground keeping Linux alove under an oppressive regime, but it isn’t realistic