The chart that Octazooka answered with is provides insight, but doesn't fully answer the question. (No offense Octa) :P
The 2019 Season's rotation for Standard is any card from the Sun & Moon expansion and after;
Whereas the rotation for Expanded is (as it has been) and card released in the Black & White expansion and after, except for the following (which are banned):
Archeops (Black & White-Noble Victories, 67/101, and Black & White-Dark Explorers, 110/108), Forest of Great Plains (XY-Ancient Origins, 74/98), and *Lysandre's Trump Card (XY-Phantom Forces, 99/119 and 118/119).
So, essentially, you won't be able to use your XY cards in Standard Formatted Tournaments, but you can use them in Expanded Formatted Tournaments, unless they are the aforementioned banned cards.
Hope I Helped!