Nate, I know you guys are neck-deep in the new release, but with the new code, bones that have zero for scale in their set-up pose no longer work at all.
For example:
Go into the Editor, new project, in Setup mode and set a 0, 0 scale on "root".
Go into Animation mode and keyframe scale at frame zero.
Now try to set a keyframe to scale 1, 1 at frame 20.
Drag the time around. The scale boxes are blank, and it doesn't have the expected result at any time.
Just a recommendation: you could use the "spine" version number in the "skeleton" data entry to provide backwards compatibility with old data. For example:
"skeleton": { "spine": "1.9.17", "hash": "TQSnS....
^^^^^^
The run-time could support migrating 1.x versions of scale timelines at run-time. Not ideal for performance, but it let's developers decide how important that is vs. re-exporting all their Spines.