Whether or not Google cares about root isn't really relevant.At this point, on Android 11, you can even apply Magisk patch during the build, and before the generation of vbmeta. So, when your rom is built, no Magisk flashing is required, which means, bootloader could be locked with custom key and you can have prerooted rom, which will pass all verifications.
I don't think Google cares about root. What it does care is that custom roms would fail Safetynet. That's why, they won't allow Magisk hide.
I did forget to mention something: This is only a problem that we have seen on a few devices, namely the Pixel 4a 5g and the Pixel 5 to name a couple. Android 12 has been released on the Pixel 3 and 4, and neither of those lines are dealing with this issue - they are able to patch and flash the boot image the same as always.
So if I'm wrong, what's your explanation for this issue that we are having on the Pixel 5? What's your solution? I'm not really interested in a custom build, but if you think you could come up with a method to easily patch /boot and /vbmeta in such a way that we will never run into this problem again, I think there's more than a few people who would be itching to try.