Not clear on why they made that call, but those are incidentally the exact set of platforms where they can't rapidly ship security updates (it's dependent on the vagaries of the platforms store).
> Mostly. Tailscale daemon client code is open source. Where the operating system is open source, the daemon and GUI are open source, and where the operating system is closed, the daemon is open source and the GUI is closed source.
You can run just the tailscaled daemons on Windows, it just wouldn't have a GUI, and that's fully open source[1].