You have to admit the topic and content of the first post is pretty SEO optimized.
It literally says "Sprite Shaders for Unity". Imagine how many people google that.
I just googled it and it's the 4th result.
Which is apt, because it's actually not Spine-specific. He actually uses it for more than just his Spine stuff.
I don't call the shots regarding who contracts who. But absolutely, anyone who uses this should credit him with big bold text in their game. It should be the first logo when the game first boots up. 'cause he shares his Sprite shaders to the world freely.
The current spine-unity 3.5 unitypackage is actually packaged for Unity 5.4 and includes the shaders in a 5.4 compatible state, though it probably doesn't handle some of the gotchas like platform-specific depth or normal direction weirdness, if any of that exists.