And how is an operating system defined in that law?

Should this be handled at the BIOS level, the kernel level, the init level, the packaging level, the GUI level, the user login level, the user desktop level, or somewhere else entirely, like a derivative distribution with its own layers, some of which will be different from the base distro?

I'm asking because each of those levels are pretty much handled by different groups of individuals, groups and organisations in different jurisdictions, cultures and countries.

While we're talking about options on where to put this "feature", who is liable for it not being implemented?

You might have an opinion on where it "should" be, but I can guarantee you that there are at least as many opinions on where it should be as people you ask.

That's why the Debian Project is doing what it is.