You're right. Though to be fair once upon a time movie posters did give you a tease of the plot in lieu of pimping out the stars that are in the movie.
Although just hyping the stars and glamour has always been a thing, too, even in the Golden Age of Hollywood. The original posters for, say, Sullivan's Travels
or Night of the Hunter
don't tell you much more than that there's a hot blonde in them . . ..
Heck, check out this poster for Casablanca:
Not a lot about the actual plot there.