Why would that be needed? I visited NYC the first time as a high school student with not so perfect English as a foreign language skills in 1980 when the city was in a pretty rotten state. The last time in 2009 when the subway was at least free of graffiti and generally looked maintained better. In younger years I found huge subway stations and their connecting walkways fascinating so might have planned some trips with extra complicated changes :) I never had the feeling of getting lost although I definitely lost all sense of orientation. It was easy enough to just "blindly" follow the signs. Not sure why an app would be needed, I assume the signs have not disappeared.
(In 1980 some connections were still sign-posted as "connection to IMT", ... BRT, or ... IND lines although those had ceased to exist 40 years earlier and where not marked on the official subway map. But even that you could just guess from the "breaks" in the line numbering.)