My posting above has become obsolete in the meantime - now both the URP and LWRP extension modules provide an outline-only shader.
Select them at your Material under the name:
Lightweight Render Pipeline/Spine/Outline/Skeleton-OutlineOnly
Universal Render Pipeline/Spine/Outline/Skeleton-OutlineOnly
.
The files are located in the following subdirectory of the URP / LWRP module:
Shaders\Outline\Spine-Skeleton-OutlineOnly-LWRP.shader
Shaders\Outline\Spine-Skeleton-OutlineOnly-URP.shader