What Pokémon are available in XY, but not in later games?

Question

For example, the Pansage, Pansear, and Panpour lines can’t be found after XY.

Answer 1

Updated answer
Following the release of Legends: Z-A, (almost) every Pokemon is available on the Switch. There are no Pokemon most recently available in X and Y.

If you were curious about the answer prior to Legends: Z-A, please read my original answer below.

Original answer
You can immediately eliminate all Pokemon introduced in Gens I - IV because of LGPE and BDSP. After that, I looked through the location data of all Gen V - VI Pokemon in the Kalos Pokedex and the Friend Safari encounter tables and found the following:

Pansage/Simisage
Pansear/Simisear
Panpour/Simipour
Patrat/Watchog

There are some that come close, like Skiddo/Gogoat prior to Gen 9, but these should be the only four lines. You might need to do some shenanigans with Mirage Islands, Island Scan, and regional forms to obtain other Pokemon in later games, but it is possible.

Comments

I forgot the watchog line existed

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these four lines are completely unobtainable in any Switch titles. Plus Furfrou as the only Gen 6 Pokemon not obtainable in any Switch titles