And what's more, the second unofficial motto of the Linux kernel is "break kernel space"; they don't make any attempt at keeping in-kernel APIs stable. So if they followed semver for userspace, they'd be on 1.9000.0, if they followed semver for kernel space they'd be on 9000.0.0.

There's value, albeit less in the minor vs patch distinction I think. So you could imagine a world in which SemVer came first, Linux uses it, and we're on 1.5.19 but usually drop the 1. because we've agreed there'll never be a v2.

That's pretty much the state of the 3.x releases, but at some point numbers got big.

I’d say it was the stage of the 2.6.x series.

In 2.6.x patch levels were the fourth number in the version, not the third.

What would be the point of that?