Generally, the rule is that if it's available through any legit means (i.e. not hacking), it's ok to use. However, you'll want to check out the rules page for specific exceptions. Each PPL goes over the rules and decides if anything needs to be changed and the rules page is updated if needed. For reference, for PPL East 2017, these Mega Stones were not allowed: Kangaskhanite, Blazikenite, Gengarite, Latiasite, Latiosite, Mewtwonite X, Mewtwonite Y, Mawilite, Lucarionite, Diancite, and Salamencite.
You can find all of the rules here:
http://paxpokemonleague.net/rules/
Alexander, the Determined Coach - East 2013
Alexander, the Emerald Knight - East 2014-2016
Alexander, the Proud Paragon - East 2017-2018
Alexander, the Rock Star - East 2019
Aurethious, the Emerald Knight - East 2020
https://www.twitch.tv/Aurethious