https://github.com/EsotericSoftware/spine-runtimes/tree/4.0/spine-unity/Assets/Spine%20Examples/Scripts/Sample%20Components/Legacy

CustomSkin.cs in spine-unity-examples, but it seems that only the .meta has been added to the repository and not the .cs.
These are samples, so there is no problem, but I will report it.

Thanks!