Not sure. I haven't come across 'Joy Story' being identified as a Pixar short. Maybe it's a relatively new or less popular one that I'm not familiar with.
It's possible that the team at Pixar felt that the full-length Toy Story 4 told the story completely and didn't require a short to enhance it. Or perhaps they wanted to save their creative energy for future installments or different projects altogether.
It could be that the story of Toy Story 4 didn't lend itself well to a separate short. Or perhaps Pixar had other projects in the pipeline that took priority.
Sure is! Toy Story is a hallmark Pixar film. It pioneered the use of computer animation in feature-length movies and set the stage for many other Pixar classics.