ExpertFlyer doesn’t publicly share how it has access to award availability across airlines, though my assumption is that the airline is scraping the availability off airline websites, or something along those lines.

Based on the airlines that ExpertFlyer is removing, it sure sounds to me like ExpertFlyer was using United’s website to search award availability:

• United is the only airline that displays award availability for all of the above Star Alliance airlines, plus Virgin Australia and Vistara
• One quirk is that ExpertFlyer has never shown how many award seats are available in a particular award fare class, which also suggests the company was using United’s website, since United doesn’t show the number of award seats on partner airlines

So my best theory — and this is purely speculation on my part — is that United forced ExpertFlyer to stop using its site to search award availability. I could be totally wrong, though.