If only there was a system that could notify customers about changes to timetables after they book, so we didn't have to be so heavy-handed
Wouldn't that be something
I don't see an issue with the idea of a feature that blocks specific trains from sale so people don't get false expectations, it should just be done via a different mechanism than fake compulsory reservations that makes it clear what is going on. It's just misleading.
LNER to be fair rarely shows a train as "sold out" as they sell a load of counted places in addition to the actual reservations, but "reservations compulsory" is still misleading as they aren't, and could be misleading the other way if someone shows up for the Caledonian Sleeper and is turned away unexpectedly because they
actually are compulsory for that.
The easiest way to do it would probably be to repurpose the hardly-used "black R" Reservations Recommended flag, rename it to "SeatSure*" with a note "tickets will only be sold for this train with a reservation; flexible tickets are valid on this train but a reservation is strongly recommended" and apply that to this sort of situation. But it would be better if NRS allowed individual trains to be opened and closed for booking separately from this - it is a useful feature, it's just that implementing it this way is an almighty bodge with lots of disadvantages.
* This is as I'm sure you know already (but for the benefit of others) LNER's name for the policy and isn't a bad way of explaining it.