alt Hacker NewsI am not aware of a case where they disabled an already-playable game via a firmware update.
But they do require certain firmware updates to play games, at least they did in the PS3 days. If you hadn't updated to that firmware (say, because you didn't want certain features you used like the OtherOS installation to be deleted) your new physical media would not play. I bought Dark Souls on a disc and could not play it on my console.
AFAIK, every game that required a Firmware update included it in the PS3_UPDATE directory in the bluray, and it being there was part of the certification process. Maybe Dark Souls was just a rare error?