Well, PID is not used anymore for nothing except shiny but any pokemon from previous generations can be considered illegal after chaning its PID or IVs if the same change would make it illegal in its origin generation.
That means a gen 3 or 4 wild pokemon should have a valid PID-IV correlation in gen 5, 6 or 7, any other value means the pokemon have been edited. And PID-Nature correlation, PID-Gender correlation, etc, because those valued cant change for any pokemon after its creation
The exception would be any gen3 pokemon that can be hatched in an egg and is inside a normal pokeball, is impossible to determine if it was caught as wild pokemon in gen3 or was hatched from an egg.
If you want to know what values are determined by PID just check bulbapedia
PD: Well, ability is excluded too. A gen 3, 4 or 5 pokemon should have the ability related to its PID, but in gen 6 and 7 the ability can be changed in game, that means no ability-PID correlation to any pokemon in gen 6 and 7
Do you mean it's not legal to modify a Pokemon transferred from the first 5 generations in Gen7?
PID/IV relation
in Pokémon Legality
Posted · Edited by PokeKai
Do you mean it's not legal to modify a Pokemon transferred from the first 5 generations in Gen7?