I don't know how many here are anime fans or like to watch random things on Netflix, but to me the saddest death would be Carla Magnolia from the show Violet Evergarden. She wasn't an important character at all, only appearing in episode 10, and the death itself wasn't even that significant as she was a sick mother whose death was a foregone conclusion. It was everything that led up to the death and the reveal afterwards that made this so sad.
Basically, Carla spent the entire episode with Violet (a ghostwriter) getting her final thoughts and documents in order before she died. Meanwhile, Anne (Carla's daughter) is getting more and more visibly upset throughout the episode as her mother isn't spending what could be her final moments with her. Ultimately the documents are finished and Violet leaves, and shortly after Carla dies. Time passes to Anne's first birthday after her mother dies, and she gets an unexpected letter - it's from her mother, wishing her a happy birthday and apologizing profusely for missing it. For the next 50 years on her birthday Anne gets a letter from her mother, as well as on her wedding day, birth of children, etc.
If this wasn't sad enough, it kicks back over to Violet as she returns to the company she works for. Violet's background is a tragic one - abandoned as a child and when found she resembles more wild animal than human. She's found by the military and is effectively raised to be a weapon, until she's gifted to someone known as the "Major", who makes the effort to deprogram her and attempt to make her a normal human. They end up permanently separated in a war, but he leaves her with one final mission: find the meaning of love. The Major's subordinate takes over for her care and brings her to the company, where first she delivers mail as she learns to ghostwrite - and use the opportunity to learn what love is.
Anyway, as a result of what happened in the episode, this ended up being the first time Violet truly showed emotion, as she broke down crying, wondering why all of this had to happen, and worrying what would become of the girl that would grow up without her mother.