Brother, I do not know when it started but it has been in place for a while. Below is an excerpt from a 2019 article.Is this something new? A couple of months ago I sold a pistol on here and I shipped it (inside its factory case) in a USPS medium flat rate box. My FFL gave me a letter to take to the post office declaring what it was along with his 'credentials' and they accepted it without question.
The USPS recommends that long guns be mailed via registered mail and that the packaging used to mail the long gun be ambiguous so as to not identify the contents. Handguns are not allowed to be mailed through USPS. See 18 U.S.C. §§1715, 922(a)(3), 922(a)(5), and 922(a)(2)(A). Rather, handguns must be shipped using a common or contract carrier (e.g., UPS or FedEx).
(https://thefirearmfirm.com/certain-firearms-can-be-shipped-through-the-u-s-mail/#:~:text=Handguns are not allowed to be mailed through,other firearms through a common or contract carrier)