Render Pipeline Converter seems to be using a fixed list of URP shaders, even when creating a copy of the
"Universal Render Pipeline/Simple Lit" shader and changing only the shaders name identifier to
Universal Render Pipeline/Simple Lit Copy, it is listed as a built-in shader to be upgraded.
So you can safely ignore the false-positive spine-unity related list entries, clicking the
Convert Assets button should do nothing at these materials other than listing a an error icon with the error message "The selected asset was not upgraded. [..]".