TIL. I noticed that lots of GitHub Actions addons use D: drive as intended, but I actually can not find the documentation that documents this for Azure DevOps Pipelines. If you happen to remember where this is documented, could you link it? It would be great to have an authoritative source for this information. I've searched pretty heavily to try to find any information about this and struggled in the past, though it could just be Google being how Google is lately. I can't even find official documentation mentioning the D: drive beyond the variable definitions that point to it.
The closest I've ever found to a real acknowledgement is this issue with relation to GitHub Actions: https://github.com/actions/runner-images/issues/8755