"Restricted" isn't the right word for it in the USA but maybe it is restricted in the UK and other places by their governments.

Most medical insurance policies in the USA say medications can only be used for an FDA approved diagnosis. This rule is selectively applied to the more expensive medications. Medications are deemed to be "experimental" when there is no research evidence submitted to the FDA that they work for each specific diagnosis.

Older medications and the less expensive ones aren't scrutinized as much. "Off label" uses of medications is a common practice when the medication is deemed to be medically appropriate. It somewhat depends on the insurance company compared to the government.

I don't think Prednisone is "FDA approved" for PMR or any diagnosis but it is widely used.