The message doesn’t say that the option is only available on the selected route.
It’s fair the assume that passengers are going to “learn” about this and try again next time in a different route, only to arrive at a destination that doesn’t support it.
It's not as though it's hard to predict in general. These aren't like "Of 492 stations served by TfL 328 picked essentially at random take contactless but the rest do not". The situation is that all of TfL's stations and all the stations in the region they get to control even if operated by somebody else, take contactless.
There are edge cases, but, they're literally edge cases, they're at the edge. Example, suppose I'm in central London and I want to go to Amersham. That's a Tube station, it's notionally in London for this purpose so my contactless just works. How about if I travel slightly further along that line, to Great Missenden? That's no longer in London, contactless is unavailable.
But, why would I expect Missenden would work? It's not in London, it's not shown on a Tube map or a TfL London map, Amersham is (right at the top left) but Missenden isn't, because it's not in London by this definition, you have finally left.
Could I be confused because I'm on a tube train? Nope. Tube trains don't go that far, they don't go beyond Amersham on that line. Once upon a time you could get tube trains out to Missenden (we're not talking last week, this is when they were steam trains like 60+ years ago) but not any more. So I have to have boarded a full size train, probably bound for Aylesbury, or Birmingham or something, and thought "I bet this is a London train and my London fare system applies". That's very silly.
It’s fair the assume that passengers are going to “learn” about this and try again next time in a different route, only to arrive at a destination that doesn’t support it.