Background information regarding User Availability
The UserAvailability (USRAVL) API call was used at some point in the "Smart Assignments" functionality to show when the user is already over allocated or not, when providing suggestions. This API call has since been depreciated due to performance issues it caused to the Smart Assignments functionality.
Is the User Availability Call still functional?
The UserAvailability call is still functional and you can get data from it, however, it exists for getting existing data only as the tables are no longer updated by Workfront due to the performance problems.
Is there a Work-Around?
The most accurate way to get around the depreciation of this API call is to use the Resource Planning -> User Utilization tab. There you can see planned allocation, projected allocation, and actual allocation to projects.