I used the form on the website to ask what had happened to the fare estimate and within 48 hours received this response:
The Fare Estimate was removed as it does not comply to legal minimum requirements for issueing pricing to customers. We are looking at alternatives but they will always need to include the full customer itinerary as pricing depends on each leg of the itinerary.
Please contact the reservation centre of any of the oneworld member airlines to get more information on pricing or complete the itinerary for oneworld Explorer on the oneworld website to get your actual fare quote before going to booking.
I am sorry for this inconvenience.