This line in Rogue One was apparently a mis-quote by the voice actors that somehow made it into the completed film. They were trying to back-reference the line from A New Hope complaining about the famously obsolete VT-16 (an item that has no canon existence other than to be complained about).
“Without saying a word, without being called upon, and without even
looking at each other, Sam and I stood up from our chairs and walked
up to the array of microphones in front of the movie screen. Matthew
said, ‘Okay, we need some kind of stormtrooper dialogue here,
guys . . . oh, looks like you’re ready, let’s go for it.’
“Sam spoke first, and I spoke second, delivering the line about T-15s
being made obsolete. He reacted in that great trooper monotone, and
then we both performed a vocal for being taken down by rebels. The
room laughed. We did a second take, and that was it. The whole thing
went down in less than two minutes.
Stormtroopers: Beyond the Armor
and
[The picture above shows a] frame of two stormtroopers having a conversation about the obsolete T-15. The exchange was intended to set up the conversation overheard during A New Hope regarding the new VT-16. However, the Rogue One conversation identifies the previous model as a T-15, incorrectly calling out the letters as that would link the two references.
Stormtroopers: Beyond the Armor
We can assume that the VT-15 is also a non-object that troopers complain about periodically (although I've heard that the T-17 is going to be a vast improvement).