The folder named "Delivery Optimization" suddenly appears in the D drive in Windows 11 system

when installing windows 10/11 updates, this folder can be created and used for caching / storing files locally

"When Delivery Optimization is configured to use peers and Microsoft Connected Cache, to achieve the best possible content delivery experience, the client connects to Connected Cache and peers in parallel."

What is Delivery Optimization? | Microsoft Learn

usually folder sits on systemdrive. but in some cases, e.g. if empty unreserved space on systemdrive is not sufficient, it gets created on other drives.

it can be configured in windows settings here:

Windows Update Delivery Optimization and privacy - Microsoft Support

there are ways to change this folder, e.g. with local group policies, GPOs or registry keys:

Change Delivery Optimization Cache Drive in Windows 10/11 | NinjaOne