Mfcttrf

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?

I think more than one term would apply here.

From wiktionary,

flashforward (plural flashforwards). A dramatic device in which a future event is inserted into the normal chronological flow of a narrative.

This certainly applies, though as with flashback it suggests that you might experience a brief scene. I would argue that these sentences contain a vivid though spare scene, no action sequence required.

You might also think of it as an instance of foretelling on the part of an omniscient narrator, foretelling being a form of prophesy more frequently found in direct speech. It differs from foreshadowing because it's a direct statement of what the future will bring, rather than an oblique hint.

If this occurred in a way to suggest that the future events would unfold counter to characters' expectations, then it would be a form of dramatic irony, because it has been revealed to the spectator/reader.

There is an answer to a different SE question under English Language and Usage that offers a few other terms, none of which I like quite as much, here: SE: Literary term for referring to a future event in the story line