Not sure if this has a name, but I think it's best demonstrated with some examples I've seen:
Nadia and her family both considered her thereafter to be without a family, something all of them, all four, for the rest of their lives, regretted, but which none of them would ever act to repair [...] because the impending descent of their city into the abyss would come before they realized that they had lost the chance.
- Exit West, by Mohsin Hamid
The events that would deal Macondo its fatal blow were just showing themselves when they brought Meme Buendia’s son home. [...] She succeeded in convincing Santa Sofia de la Piedad that she had found him floating in a basket. Ursula would die without ever knowing his origin.
- 100 Years of Solitude, by Gabriel García Márquez
Or for a totally convoluted example I just came up with,
MC left the administrative building feeling victorious. In the not so far future, a tornado would tear through the area, demolishing the building. The town would later put up a playground in its place.
Basically, I think the technique is characterized by using "would" and describing an event that will happen in the future, typically beyond the end of the book's events.
Is there a name for this technique?