Sorry to take a detour here, but I started getting this error:

Problematic material setup at Spine GameObject (blob_skeleton):
Warning: Premultiply-alpha atlas textures not supported in Linear color space!You can use a straight alpha texture with 'PMA Vertex Color' by choosing blend mode 'PMA Vertex, Straight Texture'.

If you have a PMA Texture, please
a) re-export atlas as straight alpha texture with 'premultiply alpha' unchecked
(if you have already done this, please set the 'Straight Alpha Texture' Material parameter to 'true') or
b) switch to Gamma color space via
Project Settings - Player - Other Settings - Color Space.

UnityEngine.Debug:LogWarningFormat (UnityEngine.Object,string,object[])
Spine.Unity.SkeletonRenderer:Initialize (bool,bool) (at Assets/Spine/Runtime/spine-unity/Components/SkeletonRenderer.cs:508)
Spine.Unity.SkeletonAnimation:Initialize (bool,bool) (at Assets/Spine/Runtime/spine-unity/Components/SkeletonAnimation.cs:189)
Spine.Unity.SkeletonRenderer:Awake () (at Assets/Spine/Runtime/spine-unity/Components/SkeletonRenderer.cs:402)
UnityEditor.EditorApplication:Internal_RestoreLastOpenedScenes ()

I don't think I can do viable tests until it is solved. What happened? I was just testing different shaders and selected this one:

Now the character won't render (I disabled all render features and filters to be sure)

It's just invisible. Its skeleton animation component is showing this warning:

I tried deleting the character files and reexporting/reimporting from Spine and the error persists. What did I just break?

Misaki
I haven't touched the PMA option and it was checked (Is this the default?) Now I reexported the character without the PMA option.

I prefer not to change the color space of the project, as Linear is the default and I don't want to change values that I don't understand hehe.
Anyways, I reimported the NOT-PMA png to Unity and the error persists.
Here's the PNG, in case you need to check that it is NOT-PMA.

The warnings persist

Misaki
Alright! Now I have configured the color space as Gamma and enabled PMA back. The warnings are gone and the character renders if I use the Spine/Skeleton shader.

But as soon as I change to the Universal Render Pipeline/Spine/Skeleton shader, the character disappears:

I still have the feeling that something broke in the background, as I had no issues with the Spine URP shaders before. Any ideas?

BTW, thanks so much for your help so far Misaki

Harald
Wow, I really didn't see that coming, the shaders were not working because I had reverted to the 2D renderer and I was still trying to use the 3D shaders. Changed to the 2D ones and it displays right now. I'll continue from here.
Just one more question for now. Are there versions of the outline shaders for the URP/2D category?
I mean like this one:

Harald
I see this setup in the example scene:

How can I make the outline object be mask-interactive?

Harald That's exactly what I meant, thanks!

I have encountered a new problem now. My prototype is using custom axis sorting, so the scene sprites sort themselves using the Y axis instead of Z. All the characters and ground obstacles are in a sorting layer called onGround, more specifically in position 0 inside that sorting layer.
Now that the outline object is working, I need it to render in position 1, inside the onGround layer, so the "see through" silhouette will be visible when the character is behind an obstacle.
I've tried using the SortingGroup component, as I think this has worked for me in the past. Is that the right way to force a mesh to sort using the sprite sorting system? It is not working for me right now.

What do you think I am doing wrong?
Thanks!!

I can't delete my last post, so please ignore it. My mask was not in the wrong place. The setup is working as intended.

OK! Now I have something really similar to what I was trying to do. The problem is... I don't know why it's working! hahaha
Here's the effect in action:

The thing I'm not getting is why the shader that I use for the "silhouette" is working correctly inside the masks.
These are the shader settings:

There's no info there about Stencil Comparison, not even if I look in debug mode. So I don't know how is it working correctly with the Sprite Mask.
Here's the rest of the components I'm using to create the silhouette, based on the same components used in the outline method:

And that would be all for now, just looking for some enlightenment about why this is working before I happily move on
Thanks!

Harald

Oh, I think I found it! Is this the property?

I was only looking under the "valid keywords" and "invalid keywords" not under "saved properties", my bad. Too many new things ^^
I see that the value is set to 4 which is exactly what I needed for the effect, although I don't know why is it set to this value or even if it was me who set it accidentally. Could I change this value somehow if I needed it?

I have a follow up question too. Would it be possible to have a simpler version of this shader that, instead of modifying the texture color via a screen blending effect and an input color, just changed the color value of the texture to the same input color? I'm not a coder, just a tinkerer, and almost had a heart attack when I opened the shader file in Visual Studio :_(

Anyway, thanks again for your help and your compliments Harald. The game is still in pre-production and I'm the game designer, so I'm exploring the possibilities of what we can do and what we can't. I'll make sure to keep posting some showcase bits, as the Spine community is always so kind and appreciative of the works of others.