My understanding is that if Lit use the Openreach fibre, this is laziness. They're supposed to use their own fibre, leaving Openreach's fibre alone. Therefore, the Openreach ONT would remain in-situ, in case of a return to Openreach-provided services in the future.
If Lit have used the Openreach fibre, the Openreach ONT could be disposed of, as if a return to Openreach-supplied services is required in the future, Openreach would have to reinstall an ONT, and they'll more than likely ask Lit what the hell they did with their ONT (i.e. not your responsibility)