Pixels are consistently "third party Android builds friendly", plus GrapheneOS has a list of required security features (beyond their control): https://grapheneos.org/faq#future-devices

e.g. first one in the list:

> Support for using alternate operating systems including full hardware security functionality

GrapheneOS wants users to lock the bootloader (≈enable Secure Boot) after install by providing user signing keys (avb_custom_key) -- that already seems to leave only Pixel, Nothing and Fairphone.

https://github.com/chenxiaolong/avbroot/issues/299