SkeletonRenderer is the base class of SkeletonAnimation and SkeletonAnimator.
It holds the SkeletonData asset, manages the creation of a Spine.Skeleton at runtime, and renders that skeleton into a UnityEngine.Mesh every frame.

This update has been released and is now in the latest unitypackage.
It features several bug fixes, minor improvements in speed and most significantly, easier customizability. The interface is mostly intact, though if you made any modifications to the code, you may have to move some logic.

This also allows the new SkeletonRenderSeparator feature, which is designed to replace SkeletonUtilitySubmeshRenderers.
You can find how to use it here: https://github.com/pharan/spine-unity-docs/blob/master/SkeletonRenderSeparator.md
There is also a new sample scene for it in the latest unitypackage.: Examples/Other Examples/SkeletonRenderSeparator.unity

SkeletonUtilitySubmeshRenderers can continue to function (you have to open SkeletonRenderer.cs and uncomment out the SPINE_OPTIONAL_SUBMESHRENDERER #define directive)

All of the scripts have been moved into the Spine.Unity namespace so you will have to update your scripts that interact with Spine's systems to include this on top:

using Spine.Unity;
using Spine.Unity.Modules; // for some specific optional scripts

More on that here: Spine.Unity namespaces

(More details to follow)