NO WAY!!!! This is completely false information!
I program CUDA applications. My own CUDA programs written for CUDA 9/10 work absolutely fine without having to recompile on my RTX3090 with only the CUDA11 SDK installed! You can download nVidia's flex physics demo app an example that uses CUDA9 and it works fine on my RTX3090. And you can also see this described in nVidia's documentation on backward compatibility: https://docs.nvidia.com/deploy/cuda-compatibility/
You are either doing something strange or purposely obsoleting prior versions for no good reason!!!
Support your software, and fix this, and stop providing misinformation to customers!!! This is terrible!