I've never heard of it for English-language movies. Was it superimposed on the main screen, or was there another screen? I've heard about some opera companies that offer English subtitles on a small horizontal screen adjacent to the stage.
Is this in a neighborhood with a large deaf population, maybe a school or college nearby? Or is this a shiny new digital theater? I would imagine that it would be easier to embed captions in a digital movie (like on a DVD) and turn them on by flipping a switch than to order a special print.
It was obviously operated by a switch as it did not come on till about 20 minutes into the movie.
We are not in the center of a large deaf community....I very rarely see Deaf people signing...our local schools mainstream Deaf students and it is a very small population...we are not near a school for the Deaf. But the theatre is very high tech...they even have machines to purchase your ticket via debit or credit card and in fact only have two cashiers on duty, encouraging patrons to use the machines.
I have a brother who is Deaf...he lives in another part of the country and he is not able to enjoy theatre movies yet but I am encouraged that maybe someday his theatres will have this feature or similar features.