According to Sony's official PR team, the Android 5.1 OTA will start going out to the original Xperia Z series and the T2 Ultra. That's the Z that was released way, way back in March of 2013. A two and a half year old phone is getting updated by Sony. Granted, Lollipop 5.1 was released in March 2015, so Sony is rather late with its OTA and the two-year update cycle deadline that most manufacturers seem to be following was still in effect in March. But the company should still be commended for following up on what is essentially an archaic device in its collection - the Z series is the first of what has now become 6 generations of flagship devices (Z, Z1, Z2, Z3, Z3+/Z4, Z5). As for the T2 Ultra, it dates back to January 2014, but it's a midrange device, which other OEMs usually shun when update times come.
So there you have it. Sony might be slow, but it's doing things right by its users. If you're an owner of an original Xperia Z series phone or tablet, or a T2 Ultra, check for an update and be patient. It might be on a staged rollout, and your operator might be slowing things down further.