Hey hey!
That's a thing I noticed too recently! Before, it was very useful 'cause you could hide a loooooot of keys on the timeline. Now it only hides the image itself on the scene, and obviously in the hierarchy. But no more on the timeline :S
It was previously working on any version I had used, but now since 3.6.52, it vanished. I didn't see something on the changelog about this 🙁