suanLoBeach I didn't know about this asset before, but I don't think it will have much synergy with Spine. First of all, the Spine skeleton can be animated without using Unity's state machine, and the code required to animate the skeleton is very simple. It is also possible to blend animations by using multiple tracks, or have AnimationState mix the previous and next animation at transition. There doesn't seem to be much benefit to purchasing this asset to manage the animations of the Spine skeletons.

@suanLoBeach No, the mentioned asset likely requires Unity's avatar system for placing feet, hands and so on. Spine does not use any of this, also it's working in two dimensions, not three, so I don't know how that should be transferrable. What would you want to use it for?

If you think you need it for IK foot placement, you can simply use the
SkeletonUtilityGroundConstraint component that comes with the spine-unity package. Take a look at the Spine Examples/Other Examples/SkeletonUtility GroundConstraint example scene to see it in action.