Jack's death has been a dark specter waiting in the wings on This Is Us since early in the show's run. The audience knows his death is coming, but not when or how. Jack's impact on his family has been obvious no matter what point in the timeline the show is exploring at any given moment; his advice, his sense of humor, and his innate goodness are all visible in his children. It's painful to know that he didn't live to see them grow up, even as he lives on in them. Now Jack's funeral has been shown on This Is Us and it is as heartbreaking as you would expect.
In "Three Sentences," Kate decided to forgo her gastric bypass surgery in the wake of Toby's medical issues because it just wasn't the right time for it anymore. Her doctor suggested a retreat instead that would help Kate change her lifestyle in the way she wanted, though it would be hard work; that was something Kate found out as soon as she turned up at the program. It wouldn't just be about diet and exercise: it would be about getting to the heart of her issues and untangling the the things that caused her pain. This naturally led to Kate reflecting on Jack's death, which had such a profound influence on her life.
While in the midst of an exercise that was designed to dig deep into the motivations of the retreat attendees, Kate flashed back on three of the most painful moments in her young life. One was when girls at the pool passed her a nasty note about her weight; another was when her birthday party backfired and left her alone, all of her guests drawn to Kevin instead. The third was Jack's funeral.
Though the episode still didn't reveal how Jack died, it did situate his death more clearly in the show's timeline. The pictures of him around his casket showed the Jack most audiences had come to know: still relatively young, with his signature mustache. And the children gathered by his grave were teenagers, which makes sense because viewers saw previously that Jack was alive at least until the triplets went to high school. That gives the audience an idea of when Jack's death will occur.
The brief glimpses of the funeral almost made it seem as though This Is Us was preparing the audience for things to come. It's painful now, but at least fans got a little bit of warning about the tough times ahead.