I’m not sure what you mean – I can see ‘terrain’ as an option in the ‘map’ dropdown where you select the map type. It still seems to be present in both the backend editor and the frontend.
Are you using a custom style? If so, those can only be applied to ‘roadmap’ maps. Google also released a new API version yesterday which seems to have a lot of internal changes (and some bugs), so it’s possibly related to that.