After creating a brand new solution and adding a nuget package to it, and then opening up the file packages.config, I get a warning. See enclosed.

This is not a roslyn issue. The xml editor does this whenever you open an xml file that it doesn't have a schema for.
That's not an OK customer experience. From an implementation perspective, I would ask why there is no schema. From a customer perspective, I would ask why a VS-generated file produces warnings. The customer experience is confusing and makes no sense.
If this is not a Roslyn issue, please open an appropriate bug elsewhere and close this as a duplicate.
I cannot verify that this is resolved. The issue remains in the product and there is no work item elsewhere to address it.
Adding label so this will be included in the to-verify query.
Why was this issue closed?
Because it has nothing to do with Roslyn.
Most helpful comment
That's not an OK customer experience. From an implementation perspective, I would ask why there is no schema. From a customer perspective, I would ask why a VS-generated file produces warnings. The customer experience is confusing and makes no sense.
If this is not a Roslyn issue, please open an appropriate bug elsewhere and close this as a duplicate.