Sets are usually in print for approximately two years after their release date and then Pokemon stops printing them. We saw some Standard-block sets be reprinted this past year, but the safe bet is anything not in Standard (all sets before BKT) will not be printed anymore.
You can get info on exactly when sets go out of print from big sellers like TCA Gaming, Deriums, etc.
It’s always save to assume the 6 latests sets are still in print, unless distributors got a heads up.
This because those are the sets that are legal to play in the Standard Format. And TPCI wants those cards available to everybody to prevent only rich players able to afford the best meta decks.