You can use MSAA so images cut by clipping or mesh edges are antialiased. This is not a feathered edge, but it is a lot nicer than a hard edge, so I wanted to make sure you know about it. Enabling MSAA in Spine will help in the editor and for image and video exports. If you are using a runtime, you would need to enable MSAA there to get the antialiased edges.
There is some complexity to clipping with a feathered edge (most likely it would be done using an image for the mask), as it involves shaders. Some game toolkits don't support shaders, but we could simply say the feature is not supported there. For the rest of the game toolkits, many games want to use their own shaders. When Spine needs special shader functionality, it can conflict with the game's shaders. There's not a great way to resolve that. Often the game developers need to merge their shaders with what Spine needs.
All that said, it would be a very useful feature and we would like to add it. So far we have not found time to do so, given the other features we want to add first.
Technically you could place images in the Spine editor and use your own shaders at runtime to perform clipping. Otherwise as a workaround sometimes you can use images to mask the edge of the clipping. Clipping is expensive so ideally you can hide everything behind images and not even need clipping, but of course it's not always possible.
