pixelmeat Ah, sorry my explanation was not complete. If the texture and material settings are not set appropriately for the Straight alpha texture, the error will not be resolved. See this section of the documentation for the correct settings:
Correct Texture Packer export and Texture and Material import settings

pixelmeat

I haven't touched the PMA option and it was checked (Is this the default?)

Yes, the PMA option is enabled by default. This is because there are some advantages to using PMA textures.

• Using PMA textures can reduce draw calls when using the Additive blend mode for skeleton slots.
• Better results when creating mip maps.
  To understand this, watch the following video from 1:45-4:50:
  

@pixelmeat And note that the Universal Render Pipeline/Spine/Skeleton shader is for use with URP without a 2D Renderer, while Universal Render Pipeline/2D/Spine/Skeleton is for URP when using a 2D Renderer.

See the documentation here:
https://esotericsoftware.com/spine-unity-rendering#URP-Shaders-Extension-Package

pixelmeat 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.

Glad to hear you've decided to switch to Gamma color space! This will make many things behave more intuitively.

@pixelmeat Very glad to hear you've found the issue!

pixelmeat Just one more question for now. Are there versions of the outline shaders for the URP/2D category?

It's a bit more complicated currently, unfortunately there is no single-shader version available yet. This is planned in this issue ticket:
EsotericSoftware/spine-runtimes1824
However in the meantime you can use e.g. the RenderExistingMesh component and the Universal Render Pipeline/Spine/Outline/Skeleton-OutlineOnly shader as described here in the documentation.

You can find an outline URP example scene Outline Shaders URP.unity in the Samples of the Spine URP Shaders UPM package (installed via Package Manager under the Samples tab), there you can see this setup in action.

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

By mask-interactive, do you mean the masks in the way of Inside-Mask Outside-Mask options of the Mask Interaction parameter of SkeletonAnimation? If so, that's the Stencil Comparison parameter (_StencilComp in the shader and in the Material properties under debug mode). Values are as follows (as names and parameter int-values):

• None - Inside and outside (Default): Always, 8
• Inside: LessEqual, 4
• Outside: Greater, 5

The integer values for each comparison operation can also be found under UnityEngine.Rendering.CompareFunction.

pixelmeat 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.

Are you sure it's not there in debug mode, named _StencilComp? If you are switching from a previous shader, the keywords are carried over.

Your game looks so cute BTW! 8)

pixelmeat I was only looking under the "valid keywords" and "invalid keywords" not under "saved properties", my bad. Too many new things ^^

Keywords are for switching between shader variants (active shader branches), properties are actual value paramters.

pixelmeat Could I change this value somehow if I needed it?

Yes, you see it right in front of you. You can click on the 4 and replace it with e.g. 5 .

We have just exposed the Stencil Comparison parameter for all URP blend modes shaders (via this commit). These were indeed hidden as they don't use the same custom shader GUI code as built-in-RP blend modes shaders which exposes these properties. Thanks for reporting.

A new Spine URP Shaders UPM package is available for download:
https://esotericsoftware.com/spine-unity-download

pixelmeat 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?

What do you mean by the last half of the sentence "just changed the color value of the texture to the same input color"? Do you want to output just the tint color in additive blend mode?

In general you can quite easily modify existing shaders if you just need to output a constant color. You can create a copy of the .shader file, and rename the name in the first line of the file to your desired name to be displayed in the Materials shader property.

Then you can change the line #pragma fragment frag which defines the following method from the included file "../../Include/Spine-Skeleton-ForwardPass-URP.hlsl" to be used:

half4 frag(VertexOutput i) : SV_Target{
	float4 texColor = tex2D(_MainTex, i.uv0);
#if defined(_ZWRITE)
	clip(texColor.a * i.color.a - _Cutoff);
#endif
#if defined(_TINT_BLACK_ON)
	return fragTintedColor(texColor, i.darkColor, i.color, _Color.a, _Black.a);
#else
	#if defined(_STRAIGHT_ALPHA_INPUT)
	texColor.rgb *= texColor.a;
	#endif
	return (texColor * i.color);
#endif
}

Now you can replace the used fragment program function like this in the .shader file:

 #pragma fragment yourFunction

and below the include statement add your function:

#include "../../Include/Spine-Skeleton-ForwardPass-URP.hlsl"

half4 yourFunction(VertexOutput i) : SV_Target {
	float4 texColor = tex2D(_MainTex, i.uv0);
#if defined(_ZWRITE)
	clip(texColor.a * i.color.a - _Cutoff);
#endif
	return float4(i.color.rgb, texColor.a * i.color.a);
}