Piety was a main stat (purple) that increased MP, but was changed to a healer-specific substat. Given role stats were added in Stormblood, it must have been in Stormblood.
Every job had seemingly random amounts of MP and although it increased with level, so did the usage, so it was utterly pointless. Meanwhile TP was fixed at 1,000 for everyone. Since they were removing TP, they figured to make MP work the way TP did by giving everyone a fixed 10,000 MP. With that change, they had no choice but to change how Piety worked to instead adjust the regen rate. That was in Shadowbringers.
Everyone could meld primary stats back then. I was still melding them in Stormblood to increase Strength, so it must have changed in Shadowbringers.