Thought I'd stumbled upon a multiple cities workaround but functionality seems to be gone.

Clicking through a dummy 'Today's Prices' booking and looking at the availability calendar, you can still see 'Multiple Cities?' below 'Find My Flight'.

The multi-city page loads but inputting an itinerary just yields....

Requested action is not supported. Please try again.

Tried Chrome, Safari & Firefox.

Direct URL if someone has more luck than me.