[Battle Grounds spoilers]
So, in Battle Grounds, Ethniu states that she knew Mab prior to becoming the Queen of Air and Darkness. And as part of her vendetta against Mab, she made it her goal to destroy the Unseelie Accords and everyone who stands by them. This makes it clear that the Accords are not something that have always been around, they were created during the era of human mythology, by the current Winter Queen herself - not the previous one. Which brings a question to my mind:
Why is it that Mab is the one who founded the Accords, and not Titania? Summer's purpose is to counter Winter any time there is an action that is outside the realm of opposing Outsiders and defending the Outer Gates, so wouldn't it have made more sense to have Summer be the ones to make the Accords as a way of applying a check to Winter's "extracurricular" activities? Or am I missing something?