Four years ago United removed the ability to see Singapore Airlines award space on their website. When most airlines are trying hard to get partner awards bookable online so members don’t have to call, United made the choice to take away access to Singapore space online so members would have to pick up the phone (and likely so members wouldn’t even know it was an option).
At the time United claimed this was based on an agreement they made with Singapore Airlines. Singapore called this out as a lie saying the decision was United’s and one “we have no influence over.”