Posting an updated answer since the old one is outdated.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 except Kecleon 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.** The unobtainable Pokemon are:

 - Zen mode Darmanitan (Unovan and Galarian, base modes are available)

 - Larvesta + Volcarona

 - 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

 - Hawlucha

 - 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)

 - Cutiefly

 - Wishiwashi

 - Mudbray

 - Bounsweet

 - Passimian

 - Sandygast

 - Pyukumuku

 - Type: Null

 - Minior

 - Komala

 - Turtonator

 - Bruxish

 - 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)

**Hisuian Pokedex.** The available Pokemon are:

 - Ursaluna

 - Overqwil

 - Sneasler

 - Hisuian forms for Growlithe, Arcanine, Voltorb, Electrode, Qwilfish, Sneasel, Braviary, and Avalugg.

**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, and Victini are available through special quests without a time limit. Hence the **unobtainable Pokemon** that have previously been released are:

 - Deoxys (all formes)
 - Darkrai
 - Shaymin (both formes from Go 2022 Fest)
 - 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 formes)
 - Zarude (must have started Special Research quest from 1st to 10th Oct 2021)