The Pokemon franchise has issued a definitive ruling on what is and what isn't considered to be a Legendary Pokemon.
Yesterday, The Pokemon Company announced a new yearlong promotional campaign to celebrate Legendary Pokemon. The big announcement was monthly giveaways of different Legendary Pokemon, but they also launched a new website with different Legendary Pokemon-themed activities and information. The website also definitively answers whether Type: Null, Silvally, and the Island Guardians of Alola are considered Legendary Pokemon in the official canon of the Pokemon franchise.
Since Pokemon Sun and Moon came out in 2016, Pokemon fans have debated whether any of these Pokemon are "officially" Legendary Pokemon. For those who haven't played Pokemon Sun and Moon (or their remakes Pokemon Ultra Sun and Ultra Moon), Type: Null is an artificially Pokemon made by the Aether Foundation in an attempt to create a Pokemon that can fight Ultra Beasts. Type: Null uses the RKS System (named after the mythical creator of the universe Arceus) to change its type. Eventually, Type: Null evolves into Silvally when its bond is strong enough with a trainer.
Players can't capture Type: Null in the wild in Pokemon Sun and Moon and don't have access to the powerful Pokemon before they become Pokemon champion. Gladion gives the player a Type: Null after they beat the game's main storyline.
Because of the unusual circumstances around how Type: Null was created and how it can be obtained in Pokemon Sun and Moon, there was a fierce debate as to whether it was truly a Legendary Pokemon. Even though the Pokemon was classified as a sub-legendary Pokemon in Pokemon Sun and Moon's code (a classification given to Pokemon like Articuno or Heatran that don't play a role in the plot of the main Pokemon games,) some players still questioned whether Type: Null should be a Legendary Pokemon as it evolved, something Legendary Pokemon couldn't do until Pokemon Sun and Moon.
However, the Legendary Pokemon website does include Type: Null and Silvally as Legendary Pokemon, along with the Island Guardians Tapu Koko, Tapu Lele, Tapu Fini, and Tapu Bulu. The Island Guardians were never definitively named as Legendary Pokemon in Pokemon Sun and Moon, even though they fit just about every other Legendary Pokemon category. Cosmog and Cosmoem, the pre-evolved forms of Legendary Pokemon Solgaleo and Lunala are also included on the webpage.
Interestingly, none of the Ultra Beasts appear as Legendary Pokemon, despite their fearsome stats and the fact that they too were categorized as sub-Legendary Pokemon in Pokemon Sun and Moon. The Ultra Beasts likely receive their own category as they're both interdimensional Pokemon and can be caught multiple times in the games.