Yeah, graphene devs have already released a statement saying that moving forward, their work is going to be WAY harder to complete with the same frequency as before.
Discussion
I never understood their project, why do it the hard way? Why not base it on top of lineageos and actually support many devices? I never used it not because I had no interest in it but because unlike most custom roms it wasn't available.
It will be harder for lineageOS also. The changes Google have made to their workflow will effect all ROM developers, from what I've understood.
GrapheneOS have explained before why they don't operate like lineage you can probably find it if you search their forum. But they are pixel only because Pixel phones are the only phones which allow to lock the boot loader after installing a ROM and therefore provide greater security, plus by choosing just one phone they are able to streamline security patches for optimum timely updates.