I can't speak to the Kia specifically, by my EV (Ford lightning) comes with a 10 year warranty on the battery.
Unless you don't have access to charging at home, DC fast charging is exceptionally rare. When I leave home, I have 300 miles before I need to use a DC fast charger, so I think I've used a DC fast charger around a dozen times. That's probably average, and would have minimal impact on battery health.
If you're always fast charging though, that could cause problems, don't quote me but I think I saw something talking about ten years of simulated fast charging cut 14% off the battery life. I wouldn't be surprised if 15 years show similar since battery pack failure is a bathtub curve.
How's the battery health after 15 years ?