We’ve added an updated list of hardware requirements:
https://grapheneos.org/faq#future-devices
GrapheneOS on the Pixel 8 is the first platform using ARM MTE in production and it’s such a massive improvement for exploit protection that it has to be considered a hard requirement going forward.
They don’t support it because it lacks the bare minimum security features required so a phone can be considered secure. At some point they even shipped phones with bootloader keys that were development keys and another version where re-locking could result in a broken phone half of the time. That’s why GOS isn’t supporting the FP.