I have a very recent ubitx v6 running the stock 6.1 software with the 2.8" TFT. I tried to get the KD8CEC sketch to work, but after trying all the display settings available, I could only get a blank white screen after the update. Since none of the settings mention the TFT and the code never refers to anything beyond v5, I concluded that the KD8CEC wouldn't work on a new version.
I found another port at https://github.com/reedbn/ubitxv6/. This one compiled just fine and plays with the 2.8 TFT.
What .h file settings did you use to get the CEC working on your rig? Do you have the 2.8 TFT?
I'm doing everything using the Arduino IDE 1.8.12 on linux.