This attribute is deprecated and removed from the API. See here:
The relevant part is this:
We will be dropping the endpoint /srs_stages and the attributes assignment.srs_stage_name , assignment.passed , reviews.starting_srs_stage_name and reviews.ending_srs_stage_name on August 17, 2020 . This is a full two months to make the necessary adjustments.
Open Framework may still have data in cache from before the removal date. No data is available for recent items.