When you start the game, you literally get a full screen banner that says a PSN account is required to play and you must sign in. It's just that until now you could click cancel and the game would still launch.
How can anyone call it "hidden" is literally beyond me. It's not hidden on page 527 of the EULA or something - it's literally said explicitly on a full screen banner shown to every player who has started the game. Not to mention it's mentioned on the Steam store page.