Eventually the SkookumScript team may do integrations of SkookumScript into more engines, for now we have been focusing on UE4. We won't have the bandwidth until the team grows more. However, many if not most of the SkookumScript enhancements we make for UE4 are also applicable to other engines.
SkookumScript was originally used on custom engines such as the one used by Sleeping Dogs.
Also there is nothing stopping people from integrating SkookumScript into other engines right now. Some people have already been tinkering with SkookumScript and other engines. I've heard of people getting an initial SkookumScript integration into an engine up and running in just a few hours of coding. Obviously getting all the bindings for an engine will take time unless an engine is already designed to hook into a scripting system.
You can see other discussions on using SkookumScript in other engines in this forum category: