The original developer is now "setting it free" because of a lack of time to support it.
Plus there are bigger visual shader editor projects for Unity now, including the upcoming built-in one.
But Shader Forge is purely an editor tool and compiles shaders into regular Unity shaders so just playing around with it just to learn how shader code works, or to make one shader you need and then remove Shader Forge from your project, those are actually fine uses.
Announcement tweet thread: https://twitter.com/FreyaHolmer/status/990952918076547076
Free Unity Asset Store link: https://www.assetstore.unity3d.com/#!/content/14147
Github repo: https://github.com/FreyaHolmer/ShaderForge
If you're curious about what Shader setups are compatible with Spine-Unity, check out the doc on it :
Feel free to give us feedback.