Apple released a notice to iPhone customers in San Francisco. Apple notes that "With your Clipper card on iPhone or Apple Watch, you’ll be able to ride on all 24 San Francisco Bay Area transit agencies, like BART and Muni, with Apple Pay in just a tap — no need to use Face ID, Touch ID,* or touch a vending machine or card reader.

To sta informed, you could sign up on Apple's special webpage  to receive an email when it becomes available.