Yes you read the title correctly. The first and so far, only mainline Pokemon games they are playable in are Sword and Shield. Generation 8 games.

>B-But Anon, they're playable in Pokemon Go and Let's Go!
Cross gen mobile app and a spinoff game

>B-B-But Let's Go isn't a spinoff! It's mainline! It uses all the same battle mechanics!
Let's Go is a spinoff, Colosseum and Gale of Darkness used the same battle mechanics as the mainline games, and yet those have always been considered spinoffs. Let's Go's method of capture is the same as Pokemon Go's, it's a spinoff designed as a segue for Pokemon Go players to get into the mainline games. The fact that you can use Meltan and Melmetal in LGPE makes no difference. You could use Bonsly in one simulated fight in Gale of Darkness, does that make it a Gen 3 Pokemon? No, of course it doesn't.

>B-But you can connect LGPE with Pokemon Home, meant to store all your mons from the mainlien games!
You will be able to do the same with Legends Arceus, is that a mainline game? No, it's a spinoff.
Also Pokemon Go works with Home, and that's never been considered mainline.

>Okay but Ash used Meltan and Melmetal during the Alola arc! It appeared in the anime about Gen 7!
Weak argument, Pokemon from future generations always appear in the anime. Togepi appeared during the Indigo League, Pokemon 2000, the movie starring Lugia came out during the Orange Islands arc, Marill and Donphan appeared in the first Pokemon movie, Lucario and the Mystery of Mew came out during the Hoenn arc. FUCKING HO-OH APPEARED IN THE VERY FIRST EPISODE OF THE ANIME.

Pokemon from upcoming gens always appear in the anime, it's called promotional advertising.

Any further questions? Because I think the case is settled here, it's a Gen 8 Pokemon.