Please note that the URP version of the outline shaders is a single-pass Outline-Only shader, which is intended to be rendered behind your actual object. For details please see the spine-unity documentation page here.

Did you have a look at the Outline Shaders URP example scene that comes with the Spine URP shaders UPM package? If you need to render the outline around your normal skeleton, you can use a similar setup using a RenderExistingMesh component as in the Outline Shaders URP example scene.

We also have this issue ticket where we plan to support URP's (slightly different) multi pass rendering workflow in our URP shaders in the future.