I flew SJC->DEN on Frontier 1068 (codeshare with Midwest E190 AC) on Jan 28th. I had a Classic ticket and was able to buy the stretch seat for $15 from the TA. Unfortunately stretch wasn't available on my DEN-LGA A319 leg.

On the E190 only 5 of the 16 stretch seats were occupied when the cabin door closed. Then some pax tried to move into a stretch seat and was sent back being told "...these seats are reserved for premium customers...". About midway into the flight a different pax moved into a stretch seat and wasn't challenged and stayed in the seat for the duration.