TFA contains a reference to the Linux kernel, where there is a workaround for a quirk in the Apple CPUs, where in hypervisor mode they diverge from the official Aarch64 specification.

So by "vendors" it was indeed meant "some vendors" who can afford to not care much about compatibility with the specification.

It's an old comment.

iirc it's a documented feature now - FEAT_E2H0, https://support.arm.com/documentation/109697/2025_12/Feature...

And it was retroactively defined to be allowed starting from Armv8.0.

Apple designs pre-date the ID register bit for it being a thing so it takes a quirk there however.

Unfortunately I can't comment much on Apple's situation since I work there :)

Regardless what I mean to convey is that ARM would prefer such deviations to be rare, and especially for them to not be visible from userspace. Deviations will always exist if only due to hardware bugs, so contracts can only do so much.