Thanks for the feature request. This exists already but is undocumented as we’ve not finalised if it might change in the future. We’re not sure how useful it is, but since a few people have asked, I’ll share this now:
There’s an example here: https://circleci.com/gh/gordonsyme/gordon-test/1272 (expand the “Workflow workspace env vars” step).
Attaching a workspace also prints out info about what’s happening, expand “Attaching Workspace”
One thing to note is that the workspace UUID isn’t enough to work out what content you get. You also need to know the UUIDs of the upstream jobs, those are in another env-var.
If you can give us feedback on if that’s useful and how you make use of it, we can discuss whether to solidify this feature and document it.