I used the upgrade assistant tool (version 0.5.1073.11229 ) to upgrade a library project from net48 to netstandard2.0.
The result was that it correctly removed reference <Reference Include="System.Web" /> , but added a package reference to Microsoft.AspNetCore.SystemWebAdapters.CoreServices version 1.4.0.
Microsoft.AspNetCore.SystemWebAdapters.CoreServices is not available for netstandard2.0, and so should never have been added by upgrade-assistant.
I don't have a minimal recreation, but if needed I'll endeavour to produce one.