It’s up to Spotify to provide them, Sonos doesn’t have the ability to make decisions for Spotify what they provide in the servers that the Sonos software points to. 

Are you averse to using the Spotify app for this use case?  (I am not a Spotify or a podcast user, but I am assuming that you can play podcasts direct to Sonos from the Spotify app just as you can do with music - correct me if I am wrong.)