The definitive list of non public areas would probably be in the hands of what used to be known as the LT Civils 'Bridge Engineers', those who survey and maintain such structures. Outside LUL I do not believe there is such a list. Generally speaking there literally are hundreds of NPAs across the system, disused lift shafts, disused escalator shafts, disused stations, thousands of passageways and subways totally unknown to and never seen by passengers. I worked in many of these areas over the years and without being specific I can say that many of the old lift shafts used to house Strowger telephone exchanges, one contains an electricity substation, some contain Comms equipment rooms, at least one is a sumps and pumps equipment room, another held a TD equipment room, many were converted into vent shafts, most are used as cable routes for various systems, many are capped and passengers walk over them day in day out never realising that a 60' or deeper void is literally a couple of feet beneath their own! Several are used as sumps collecting water from various subterranean areas before is is pumped away.
I know several sites with disused escalators or disused escalator chambers, one still contains its escalator hidden from public gaze, one is derelict and another is cable route.
There are places where older platform fascia and a void exist behind the fascia now seen by the public.
At Camden Town the bottom of one lift shaft contained a Comms equipment room and I believe the IMR was built partially in a lift landing subway.
The old passageways to disused lift shafts have had many uses over the years.
The disused tunnels at London Bridge are where the scientific section used to burn cables as research for new less toxic and fire resistant cable materials.
Then there are the disused substations etc, the old generating styation and boiler house at Wood Lane, now demolished I believe, found use as Material Stores, were used by the LFB to mock up a section of station and burn it in a controlled manner and the film 'Who Framed Roger Rabbit' was partly made there too.
Disused stations have found new uses as substations, shops, departmental offices, government department offices etc.
There are other areas such as emergency detrainment points at various sites and several disused staircases including the second staircase from ticket hall to lower landing which sits in the old spiral escalator shaft.