It's referring to the T-16 skyhopper.
It's the airship Luke owns in episode four.
When they say that there is a replacement they specifically say
the T-15.
In A New Hope you also see Luke playing with a model of it.
scifi.stackexchange.com/users/40833/CBredlow
I believe they're discussing the VT-16 .
This is the same topic the Stormtroopers in A New Hope discuss while Obi-Wan Kenobi is disabling the tractor beam.
Curiously the Wookieepedia article suggests this is a droid seen in Return of the Jedi.
Personally I'd dispute that as it seems an odd thing for Stormtroopers to discuss, and I'd assume that VT-16 and BT-16 are different things.
But the link above does quote the conversation from A New Hope.
EDIT:
After a second viewing they definitely say T-15.
I now believe the quote on Wookieepedia is misplaced, and that those Stormtroopers are actually discussing the T-16, not VT-16.
scifi.stackexchange.com/users/63647/SeanR
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 evenlooking at each other, Sam and I stood up from our chairs and walkedup to the array of microphones in front of the movie screen.
Matthewsaid, ‘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-15sbeing made obsolete.
He reacted in that great trooper monotone, andthen we both performed a vocal for being taken down by rebels.
Theroom laughed.
We did a second take, and that was it.
The whole thingwent 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).
scifi.stackexchange.com/users/20774/Valorum
I thought they said "C-15"; I assumed it had something to do with a Clone army and was a passing reference to the imperial army weaning itself off of clone troopers.
scifi.stackexchange.com/users/75518/Pipers_Down
It's a nod to A New Hope, "You seen that new VT-16?""Yeah, some of the other guys were telling me about it.
They say it's… it's quite a thing to see…"―KE-829
and his partner aboard the first
Death StarThe BT-16 perimeter droid, also known as the VT-16, was a bulbous, spider-like droid.
scifi.stackexchange.com/users/75567/DrStrange
I think that this is a reference to the non canon characters, Tag and Bink!
It also could be a reference to episode 4, when Tag and Bink are talking about the BT-16.
scifi.stackexchange.com/users/75641/Jonah
At the beginning of the beach part of Rogue One, two stormtroopers are discussing something that they say is being rendered obsolete, followed by 'It was about time'.  It sounded like AT18, which is not a thing. It cannot be an AT-TE, as in Star Wars: Rebels it is obsolete already. What was it?
What do the stormtroopers describe as obsolete in Rogue One?
scifi.stackexchange.com
<star-wars><rogue-one>
scifi.stackexchange.com/questions/147478
scifi.stackexchange.com/users/72099/Matteo Ferla
There is no real agreement as to what the stormtroopers believe to be obsolete at the beginning of Rogue One. However, it could be that they say "T-16", "T-15", "C-15" or "VT-16". There is also the suggestion that the line is a mistake. It is uncertain what the reference is - it could be the T-16 skyhopper for example or it might even be nothing more than a misquote by the voice actors.
It's a reference to the T-16 skyhopper. It's a reference to the T-15. It's a misquote by the voice actors. They might be saying "C-15" in reference to the Clone army. It's a reference to the VT-16 in A New Hope. It could be a reference to the characters Tag and Bink who talk about the BT-16.