Is MTE on GrapheneOS restricted to some (newest?) Pixel models? Or does it work with all models that are currently supported by GrapheneOS itself?

MTE is only available in hardware on Pixel 8 and later https://googleprojectzero.blogspot.com/2023/11/first-handset.... GrapheneOS supports all the Pixel 8 and 9 series phones. They plan to support Pixel 10 once Google stop delaying their open-source releases of AOSP.