With Legends Z-A likely to be the last Switch 1 Pokemon game
I reckon that whatever is left Pokemon wise from every generation will be obtainable, now that the Pan Monkeys, the Patrat line and Furfrou has now been confirmed, I have decided to do a breakdown of what is obtainable by Generation on the Switch:
Generation 1: 151/151 (Mew can be obtained on BDSP if the player has LGPE on Switch)
Generation 2: 250/251 (Celebi is currently unobtainable on the Switch natively, was an Event in Generation 8 if someone subscribed to the Pokemon Newsletter)
Generation 3: 385/386 (Deoxys is the only Pokemon from Generation 3 that cannot be obtained on the Switch natively. Last time it was available was ORAS in Generation 6)
Generation 4: 493/493 (Manaphy can be obtained in PLA. Shaymin and Darkrai are obtainable on BDSP via Oak's Letter and Member Card and PLA if someone has a SwSh and BDSP Save file on the Switch. Arceus is obtainable on PLA if someone catches all Pokemon on the game and can be obtained via Azure Flute in BDSP if player successfully completes the story missions in PLA)
Generation 5: 647/649 (Victini was obtainable via Event in Generation 8. Both Keldeo and Meloetta can be caught in SwSh and SV respectively. Genesect's last event in the West was in 2016)
Generation 6: 719/721 (Hoopa is confirmed for DLC, while both Diancie and Volcanion are transfer-only Pokemon)
Generation 7: 804/809 (Magearna is a HOME gift for completing the Dex, Marshadow last had an event in Generation 7, Zeraora was a limited time event in Generations 7 and 8 and the Meltan line is an awkward situation)
Generation 8: 904/905 (Zarude was an event that was distributed in the same way and time as Celebi back in Generation 8)
Generation 9: 1025/1025 (Pecharunt, the only Mythical in this Generation, can be obtained at any time until the End-of-Service)
Assuming Generation 10 is a Switch 2 exclusive, it would make sense that they would make the rest of the Pokemon that cannot be obtained natively on the Switch obtainable in some way while making the first batch of Pokemon available in the Switch 2 at the same time, especially if the Pan Monkeys, the Patrat line and Furfrou won't be obtainable in a future game until later in the Switch 2 Era, and I reckon in the Switch 2 era, I think we'll have four year Generations, with a game every two years, especially as the younger developers are now taking over the series, so I think Generation 10 will go like this:
Generation 10 and DLC (2026 for Generation 10, 2027 for DLC. Latter announced before game releases)
Unova or Johto game (2028. With Legends Z-A appearing to take place years after Pokemon X/Y, a Johto game taking place years after the originals can fix up the issues relating to Johto as a region. 2028 will be eighteen years since Black and White's release in Japan, so I think an Unova remake is likely)