Before updating a cudnn version (or any other 3rd party library) we are usually running it for a couple of weeks in the nightlies to get visibility into potential issues.
In the 1.9.0 release we were working on fixing the library split issue 9, the missing cutlass/xmma kernels 3 etc.
While cudnn8.2.1 is already released, the risk was too high to fix the outstanding bugs and update cudnn without the visibility of the nightlies, which is why it’s delayed.