Ah, I was wrong. 2.1 does have IK.
But still, it would cause unpredictable problems if you use an older Spine (especially Spine 2.x) in the latest runtime (designed for Spine 3.5)
We recommend always matching your Spine editor with your runtime, or else what you see in the editor may not be what you see at runtime. Skeletons may not even load sometimes. Your Unity console would have logged warnings and errors for you.