GrapheneOS

This won't be retro-active, that's why only Pixels are supported today: Motorola has to include some changes in the hardware to meet GOS requirements, and they are doing that, at least on some of their devices. No other phone maker other than Google has met them fully so far.

One can only hope it's successful enough to incentivize some others to go ahead and meet these requirements: Graphene is not a company and will certainly happily work with other good willing phone makers.