The backstory is reality. For example in Fritz Leiber’s The Wanderer, a planet-sized space shape crosses hyperspace and emerges in orbit around Earth. The moon is ripped apart, tidal waves and earthquakes ravage the world and the characters struggle to survive. Plus, of course, there are aliens.
Up until the starship appeared, the world was normal. We don’t need to know what it was like before the start of the story because we were living in it (we do get some backstory later on the spaceship and its inhabitants). The backstory is irrelevant.
I come close to this with Atoms For Peace: even though the world is slightly off-kilter (recovering from a Martian invasion) it still seems like that was one crazy fluke. Then Gwen Montgomery discovers a mutated lizard man dead in her street …
The protagonist is a newbie. This is one specfic uses a lot: the POV character is thrust into a new situation knowing nothing about the backstory. This excuses them asking constant questions and sitting through infodumps in response. This is painful to read if the info dump isn’t interesting (it usually isn’t). One of the things I hated about Charles Stross’s The Family Trade was the constant stream of infodumping directed at the protagonist. It doesn’t have to be a problem, though, if it’s done well: Mur Lafferty introduced a newbie to the supernatural world in The Shambling Guide to New York City without leaving me feeling dumped on.
In media res. This is the one I tend toward in my own writing — the protagonists aren’t newbies and whatever’s going on has been going on a while.
I’m not so much talking about starting in the middle of the action (which I do sometimes) as much as establishing that the weirdness pre-existed the events of the book. In No One Can Slay Her, for instance, magic’s a part of every day life in the 1950s. Jennifer Armstrong has been dealing with supernatural threats since her teen years (her wyrd guarantees it); her Beatnik wife Kate has the gift of wild magic. When I wrote Brain From Outer Space (the as yet uncompleted novel that inspired the Atoms for Peace stories), alien invasions, pod people, mutants and mad science were just “Tuesday” for my cast.
It’s common in urban fantasy, which Gail Z. Martin writes, so it’s not surprising she and her husband went that route in their steampunk fantasy Iron & Blood (cover by Michael Kormarck, all rights remain with current holder). Jake and his partner Rick have been relic-hunting for a while (mostly stealing antiques from people whose ownership claim is dubious). Steampunk tech is taken as normal, magic is middling (not everyone believes). And the events that trigger the plot — Jake’s father acquired a rare item that someone wants enough to kill him (and they did) — have been accomplished before Page One. We get some exposition about the characters along the way, but not much about the setting.
I enjoy that approach. Like I said, it’s one I use a lot myself. Although I found having the two federal agents “Sturm and Drang” already hunting a Jack the Ripper type as the book starts made it a little overfull (perhaps it’s because the Martins are going to spin them off into their own adventures). I still really enjoyed the book (and that is my honest opinion, even though Gail’s a friend of mine).