On July 15, 2019, Scryfall will be removing support for the Frontier format.
Keeping format data current is a nontrivial task for the Scryfall team. Frontier has not recieved enough momentum and excitement from the community to continue maintaining it.
Scryfall API objects will lose the legalities[frontier] property on July 15, 2019.
If you have questions about this update, please don’t hesitate to contact us.