Is the only other way to accomplish this without alpha cutoff to separate the blast animation into a separate Game Object?
No, you could also put it on another atlas page so that it will be in a second separate material. At this material you can then change shaders and shader parameters, etc.
I've reduced the alpha value of the slot attachment of the blast sprite to about 50% and it's still displaying a shadow.
What I meant was to change the alpha value in the texture, but changing slot color is also a valid approach.
Unfortunately you just discovered a bug there, the shader does not take slotcolor.alpha
into account when applying the alpha cutoff. This will be fixed in the 3.8-beta branch very soon.
I have added an issue ticket here for you to track progress:
[unity] Shadow alpha cutoff ignores SlotColor.a · #1350
The issue has been fixed for all shaders in the 3.8-beta branch.
You can download the latest package from here as usual:
Spine Unity Download
In case you need the shader bugfix for 3.7, you could overwrite only the 3.7 shaders with their 3.8 counterparts in the current latest versions (May 10, 2019). Note that there are multiple Shader
directories in the Spine-Unity runtime.