Which Pokemon have never been part of a Regional Pokedex?

Question

So, I was look through Phione's page for absolutely no reason, until I realized that Phione has never been part of a Regional Pokedex before, are there any other Pokemon not part of a regional pokedex?

Answer 1

As of the SwSh Crown Tundra expansion:

Heatran
Regigigas
Cresselia
Phione
Darkrai
Shaymin
Arceus

Source

Comments

Huh, interesting how it's mostly Gen 4 Pokemon

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It's all gen 4 Pokemon.

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That's... What I meant

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Mostly=/= All

Answer 2

As of Legends: Arceus, there are no longer any Pokemon absent from a regional dex, as all the previous Pokemon excluded from it are now included in the Hisuian Pokedex.

Source