It's not a spin-off. It's an official main series title.
This Twitter post mentions that Legends: Arceus' Japanese version has the official name for the main series games. It was called Pocket Monsters Series in its Japanese trailer, and Pocket Monsters Series is the official term for the Main Games in Japan.
It's also awkward to think that Game Freak would try for a spin-off while they work on the main DPP remakes. None of the spin-off games have taken place in the same region as a main series game. Legends Arceus is advertised as the “new era” of Pokémon, which implies that it’s “better” or “evolved” than what Pokémon is now. The higher-ups of Pokémon can't allow a side game to be “better” than the main series game.
The definition of "main series" games also needs a bit of working on. Typically, a game which is compatible with HOME can be considered to be a main series game. Being an RPG produced by Game Freak isn't a strong definition, mainly because BDSP isn't produced by GF and it's a main series game. GF outsourced BDSP and spend their resources on Legends. This isn't orthodox for GF; and yet both of the games are widely considered main series only.
People say it should be a spin-off as it's different from the other main series games. GameFreak, known for being conservative and repeating the same yearly formula, took a big gamble with Legends. How the storyline progresses (i.e. gyms, starters etc.) isn't a concrete base to define main series games. This "new" era of Pokémon would probably have more games like Legends, and there would be more arguments and discussions on whether they are a spin-off or not. But with this new start, most of them probably would be main series only.
