The C9 Pro runs on a 28nm Snapdragon 653 chipset. While powerful for its time, Qualcomm stopped providing driver updates for this chipset after Android 8.x. Without updated GPU and DSP drivers from Qualcomm, Samsung cannot build a stable Android 11 kernel. Simply put: The hardware drivers don't exist.
If you are still using a stock Samsung C9 Pro running Android 8.0 Oreo, your last security patch is likely from or January 2019 . samsung c9 pro android 11 update