I believe so, it says this spine unity runtime works with 3.6.xx. I can try 3.7 though haven't gotten around to it since everything is fairly stable in our project right now haha.
Hey I figured out how to use Unity's deep profiler which gave some more information on this...

Note that this is still on Spine 3.6 export/Unity, but the code for AddSubmesh() appears to be the same in 3.7.
By debugging, it looks like meshGenerator.BuildMesh(currentInstructions, updateTriangles) is the culprit, but only if a skeleton has clipping in it as well as multiple atlases. If I turn on Single Submesh, or force clipping to off, then there is no GC alloc issues.
So for now I am going to work with our animator to remove all instances of clipping in the Spine file, and animate in any way possible without it.
It is also my guess that once we switch to doing run-time attachments/materials, then this problem would also go away...