That's right although they do appear on the platform screens on Platform 9 at Stratford and you are allowed to board them to Liverpool St. But they do not appear on journey planners, or on other departure board screens.
In the pre-Pendolino increasingly distant past, I used to aim for the 18:25 Euston-Manchester which stopped first at Wilmslow. An excellent case for a set-down only stop in case it was running early. On one particular occasion I remember it wasn't early at all because we made a special stop at Rugby to drop off a couple who had hopped on at Euston in the expectation that it was going to stop at Watford Junction.I know that all Vrigin West Coast services Pick Up Northbound at Watford Junction and Set Down going into Euston and it's always announced to!
Never have understood why Stratford insists on advertising the Norwich services in the UP direction as it only serves as yet another performance issue for the dispatch staff with slam door stock.
I've often observed with slam-door stock that someone getting on is more likely to close the door than someone getting off. Advertising it on the platform screens but not in the timetable (and by extension, not on apps etc) may mean that a handful of people get on, but its only a handful. Perhaps it speeds up door closing a little? I dunno.