It won't be enabled, it'll remain disabled for us.
When and if Google forces apps to start using the privacy sandbox instead of their own tracking, it might make sense for GrapheneOS to spoof it so apps think it's being used when it isn't, to maintain app compatibility for apps that expect it to be there, but right now, that is not relevant, it's not being used in GrapheneOS or Stock OS. it's just there for developers. It does nothing as is, and that won't change.