It has been already. Our current R5S/R5C images ship with mainline kernel.
What is still missing is a migration for running DietPi systems. I did not find time to test and implement it, but it should be possible.
For reference, somehow CPU and RAM benchmarks are slower with mainline kernel, but probably acceptable and might become better with future kernel versions: NanoPi R5S/R5C and 6 series | Move to Armbian kernel and bootloader builds by MichaIng · Pull Request #6902 · MichaIng/DietPi · GitHub