The iTunes Movie Trailers app was released in 2011, as a simple front to see HD movie trailers on your iPhone, iPad, and Apple TV. As of today, August 11, the iTunes Movie Trailers app is being phased out.
Now, rather than having a dedicated app (or website), movie trailers can be found in the Apple TV app on any device that it's on. This is a move that makes sense, as the previous app was being neglected, and it hadn't been updated in a few years. Since you'd find Movies and TV in the Apple TV app anyway, it makes sense for everything to be in one spot.
This shows that iTunes is slowly being phased out, as Apple is pushing for subscription services more and more. Apple Music is how they primarily want you to listen to music. Apple TV has only originals, but you can buy everything from the Apple TV app. iTunes hasn't been a standalone app on the Mac since 2018. The brand is fizzling away, and this is one more blow to the iTunes name.