Posting an updated answer since the old one is outdated.
Kanto Pokedex. All Pokemon are obtainable.
Jotho Pokedex. All Pokemon are obtainable.
Hoenn Pokedex. All Pokemon are obtainable.
Sinnoh Pokedex. The unobtainable Pokemon are:
- Rotom (all forms except Mow and Wash are unobtainable)
- Phione and Manaphy
- Arceus (all forms)
Unova Pokedex. All Pokemon are obtainable, but some alternate forms are unobtainable:
Zen mode Darmanitan (Unovan and Galarian, base modes are available)
Kyurem-Black and Kyurem-White (base form is available)
Keldeo Resolute form
Meloetta Pirouette form (Aria/base form is available)
Kalos Pokedex. The Pokemon listed below and their evolutions are unavailable:
Skiddo
Honedge
Carbink
Zygarde (all forms)
Diancie
Volcanion
Alolan Pokedex. All Alolan forms of Kanto Pokemon are available. The Pokemon listed below and their evolutions are unavailable:
Dusk form Lyncanroc (Midday and Midnight are available)
Wishiwashi
Mudbray
Passimian
Pyukumuku
Type: Null
Minior
Komala
Turtonator
Drampa
Dhelmise
Necrozma (all forms)
Magearna
Marshadow
Poipole
Stakataka
Blacephalon
Zeraora
Galarian Pokedex. The following Pokemon and their evolutions are available, in addition to all Galarian forms except Corsola:
Skwovet
Wooloo
Obstagoon
Persekker
Sirfetch'd
Mr Rime
Runerigus
Falinks
Zacian (base form only)
Zamazenta (base form only)
Zarude (base/non-Dada form only)
Regieleki
Regidrago
Hisuian Pokedex. The available Pokemon are:
Kleavor
Ursaluna
Overqwil
Sneasler
Hisuian forms for Growlithe, Arcanine, Voltorb, Electrode, Qwilfish, Sneasel, Braviary, and Avalugg.
Paldea Pokedex. Only Gimmighoul (Roaming Form) and Gholdengo are available.
Unknown Pokedex. All Pokemon are obtainable (Meltan and Melmetal)
Many of the obtainable Pokemon can't currently be caught, for example most legendaries. If starting now, you'll need to trade with a player to obtain these Pokemon. Unfortunately Mythical Pokemon other than Meltan and Melmetal can't be traded. Mythicals that aren't currently available cannot be obtained until they became available again. Fortunately Mew, Celebi, Jirachi, Victini and Shaymin are available through special quests without a time limit. Hence the unobtainable Pokemon that have previously been released are:
- Deoxys (all forms)
- Darkrai
- Keldeo (base form, requires purchasing a Something Extraordinary Ticket from 6th to 11th Dec 2022 and completing the Special Research)
- Meloetta (Aria/base form)
- Genesect (all forms)
- Zarude (must have started Special Research quest from 1st to 10th Oct 2021)