Question

Why are there no gen 8 or 9 shiny sprites on the webesite?

Comments

there haven't been any different sprites for the shiny pokemon. rather than use their in-battle appearance, database uses in-box/party appearance. didn't want to put this in an answer because it's a shoddy answer

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Since the transition to Switch, Pokemon haven't had in-battle "sprites", they've had 3D models. The sprites used for Gen 8 Pokemon are the PC box sprites, which don't have shiny variations

Answer 1

There are Shiny models from Pokemon Home on the site, but you need to click through to the full gallery (example) to see those.

As mentioned by others, PokemonDB has been adding menu sprites/artwork to its gallery that don't always have Shiny variants. Adding stills of the models used in 3D Pokemon games may be something Pokemaster intends to do (seeing as it was done with Gen 6), and that would likely come along with Shiny versions.