Another interesting thing about the Yandere is it may be the only Anime Archetype where one character is considered the most Iconic example of them all and yet she's not actually one of the earliest examples.
There are multiple pre Future Diary Yandere, Durarara season 1 alone has multiple Yandere, there are characters who are argued to be a "Deconstruction" of the Yandere before Future Diary came along like with School Days and Higurashi. Shuffle was already creating a Yandere where the source material didn't have one. Detective Conan has had occasional Yandere culprits since the very first episode.
Yuna Gasai didn't become virtually synonymous with the Yandere because she helped create it, her writers simply perfected an already very popular concept.
And yet people hating a show they don't like with a Yandere will call her a "Discount Yuno Gasai" because they simply assume the whole trend must have started with the most well known.