Why are some sprites missing?

Question

Looking at certain pages, like the Regis and other legends, under the Sprites section and hitting "show all sprites", there are NO sprites for Gen 7, despite the Pokémon obviously existing in that generation. Additionally, the categories for Gens 8 and 9 only show the in-box sprites, not the over world sprites, and none of these show shiny form, either. Why is that???

Answer 1

Pokemaster, (the owner of the site), used the 3D Pokedex models for the "sprites" of Gen 7, but changed course for the Switch releases to use the box sprites instead.

It should be noted that the Pokemon released during Legends: Arceus do have 3D models and shiny variants in their Pokebase selection, most likely because they didn't really have "box sprites".

The Regis "existed" during Gen 7, but were not part of the Pokedex, and therefore did not have Pokebase Pokedex sprites, even though they technically had box sprites.

There are no shiny variations for the Gen 8-9 (excluding LA) box sprites because there are none! It's how Game Freak designed the game. Box sprites are universal for each form of each Pokemon, and do not reflect the Pokemon's shininess, (outside of Pokemon Home, thank goodness).

It is arguable that PM should have used overworld sprites instead of box sprites for Gens 8-9, as Serebii was able to, but now it's a mute point. Hopefully, PM changes this for Legends Z-A and a future Generation 10.

Hope this helps!