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In the new 90210 series, the actors are now playing themselves and their former characters were just TV show characters. This comes in the beginning of the episode, and when I watched it I thought it was a neat take on how to bring both the world they portrayed and the real world together. The connections being shown now, and the aftermath of the 90210 tv show reunion between these actors were the real focus.

Has this style of storytelling been used before? If so, what do you call it?

*one movie I can think of that would probably fit this would be Wes Craven's New Nightmare. Where Robert England as Freddy Krueger, and a demon trying to exist as the real Freddy Kruegur, both exist. But that was a little different in Freddy was really in our world with powers.

I am a huge fan of this practice. TVTropes calls it As Himself and has the hilarious apocalypse comedy "This is The End" as a screenshot.

This trope is when a real-life celebrity or famous figure is playing a fictionalized version of themselves, as a main character or recurring character. This is mostly a television trope, but there are film examples. For shows that take place in an alternate Hollywood, such as Curb Your Enthusiasm, Entourage and The Larry Sanders Show, this is the norm. It would be hard to believe it's Hollywood if you'd never heard of any of the "stars", would it?

This real celebrity is playing themselves, but they are inserted into fictional circumstances, play alongside clearly fictitious characters and sometimes have fictional backstories in relation to those fictional characters. This differs from an Autobiographical Role, where the celebrity is playing themselves in the actual story of their life.

Another example was James Van Der Beek in "Don't Trust the B* in Apartment 23". My examples are comedy but any fiction really fits the Trope

In 2008, ITV aired a pair of shows that worked like this: Echo Beach, a soap opera, and Moving Wallpaper, a comedy-drama show about the (fictional) production staff of Echo Beach. The two shows aired back-to-back, with each week's Moving Wallpaper episode depicting the production of the Echo Beach episode that immediately followed it.

Actors from Echo Beach regularly appear in Moving Wallpaper, playing their real-life selves. The two most prominent would be Martine McCutcheon and Jason Donovan, who play the central characters of Echo Beach, and appear as themselves in Moving Wallpaper. Characters from Moving Wallpaper are also listed in the credits of Echo Beach, as screenwriters and so on. It was all pretty clever, but unfortunately only lasted one season.

Although viewers didn't get to explore it, because the series was just cancled (and this only beats the 90210 revival by a few months), The OA's second season ends on a mass cliffhanger where,

Prairie/The OA travels to yet another dimension/parallel universe, but
one that is not as quite as parallel, as the previous one featured
during the second season. The scene transition with Prairie levitating
into the sky, only for it to suddenly appear as thought they are
filming on a TV or movie Set! Prairie falls from an accident with the
crane that was lifting her. Jason Isaacs, who previously played Dr.
Hap on the series, and Prairie's nemesis, comes rushing to her side and it's is
revealed that he is now "Jason Isaacs", an actor that plays presumably
Doctor Hap, and in this reality he's married to "Brit Marling", who
may be "playing" Prairie.

Generally anything that exists inside (in-universe) and outside (Out of universe) of itself within one work is considered some type of "Meta" including this 90210 revival:

In BH90210, Fox's meta-revival of Beverly Hills, 90210, the series'
original castmembers play heightened versions of themselves as they
try to mount a reboot of their '90s teen soap. The hourlong dramedy is
full of Easter eggs for fans — literally, as Jennie Garth has a
nightmare about exchanging an egg — but the series is also full of
tongue-in-cheek references to the stars' real lives.

But m1gp0z's "As Himself" trope is more specific as the type of meta it is.