(Context for this answer: I can't read properly. So it's in two parts)
If you're asking what Pokemon are in XY that are not in Z-A, then here's the list:
Zigzagoon, Linoone, Caterpie, Metapod, Butterfree, Bidoof, Bibarel, Dunsparce, Azurill, Marill, Azumarill, Burmy, Wormadam, Mothim, Surskit, Masquerain, Corphish, Crawdaunt, Goldeen, Seaking, Psyduck, Golduck, Farfetch’d, Ledyba, Ledian, Combee, Vespiquen, Skitty, Delcatty, Doduo, Dodrio, Plusle, Minun, Gulpin, Swalot, Oddish, Gloom, Vileplume, Bellossom, Sentret, Furret, Nincada, Ninjask, Shedinja, Kecleon, Smeargle, Croagunk, Toxicroak, Ducklett, Swanna, Volbeat, Illumise, Hoppip, Skiploom, Jumpluff, Munchlax, Snorlax, Whismur, Loudred, Exploud Zubat, Golbat, Crobat, Axew, Fraxure, Haxorus, Drifloon, Drifblim, Mienfoo, Mienshao, Zangoose, Seviper, Spoink, Grumpig, Zorua, Zoroark, Gothita, Gothorita, Gothitelle, Bonsly, Sudowoodo, Spinda, Tediursa, Ursaring, Lickitung, Lickilicky, Ditto, Druddigon, Deino, Zweilous, Hydreigon. Yes, there may be some errors, I've had 10 hours of sleep in the last week, sorry if there is.
If you're asking about features, then the list is something like this:
XY's Turn based battle system is gone (Replaced with time cooldowns and aiming)
because of the fact that tall grass was last featured in Sword and Shield, there are no horde battles.
some social mechanics like Friend Safari are gone.
