While it's true that volatile-pay work is extremely stressful, this seems like a cherry-picked study to be able to write a flashy headline to people rightly concerned over a hot-button issue. They could have just as easily just said that volatile pay causes anxiety and stress, which IMHO you don't need a study for, just like we don't need a study saying that eating less increases hunger symptoms.
Anxiety and stress itself is a well-known cause of all these above issues, and there are a multitude of other work-related stresses than just volatile pay.
Because in my view their problem wasn't about API versioning. They came out with a new version and deprecated the old, after all. It was that they provided way too much coverage early on.
Just because you or anyone working there doesn't see a useful use case for using that functionality over the API doesn't mean there won't be someone who will come up with something useful based on that in the future.