The T-15 was used by the Galactic Empire until it was rendered obsolete.
During the Mimban Campaign, a swamp trooper reported to another that the T-15s were down, meaning they would have to walk.
The T-15s were rumored to be marked obsolete by the time of the Battle of Scarif, as discussed by two stormtroopers right before being knocked out by Chirrut Îmwe. They implied when discussing this that they were looking forward to them being designated as such.
The T-15 mentions in Rogue One: A Star Wars Story and Solo: A Star Wars Story were added by voice actor Sam Witwer as a reference to a conversation in between two stormtroopers who are discussing the BT-16 while Obi-Wan Kenobi disables the tractor beam. Witwer also added similar lines in the sequel trilogy, which mention the T-17 and T-18. A similar reference to the T-16 was added in "Part III" of the series Obi-Wan Kenobi. The mention depended on the era.