Is the new 5e Piety system restricted to only the deities of Theros?

The new sourcebook Mythic Odysseys of Theros includes the Piety system that allows the player to earn buffs and abilities depending on their relationship with their deity. The book defines all of this for the deities added in the update, but I was wondering if there are official rules for applying Piety to the existing Pantheon. I’m sure if there’s not, there will be homebrew, but just checking if the official rules contain it and I’m just missing it.