I don’t know what the official answer is but, as far as I know, Motability don’t supply second hand adaptations, so presumably they are removed prior to selling off the car and disposed of? Personally, I would remove it when you return the car and in the highly unlikely event that they subsequently ask for it you could return it then.
I will also be interested to know if there’s an official policy on this, as we have a person hoist and if we leave Motability at the end of the lease I might pay the installer to set it up in a private car, as they list a kit to transfer it as an option.