They said on Twitter that future devices in the Razr (foldable) and signature line will be supported. The current devices by Motorola do not fulfill their hardware requirements, so no need to buy one yet. This is speculation on my part, but its not unthinkable that non-flagship support could happen eventually, although mid tier SoCs generally don't have the hardware required to support graphene (hardware memory tagging, sufficiently open secure element, etc), so in the medium term, it's unlikely that anything but the flagships will be supported by graphene.