Built-in SpeedTree Billboard shader is not supported?

I've been using a customized version of SpeedTreeBillboard.shader in my project for over a year now. With 5.6, I now get this error: "Shader warning in 'SpeedTreeBillboard': Shader is not supported on this GPU (none of subshaders/fallbacks are suitable)"

In trying to debug the issue, I tried importing the official, built-in SpeedTreeBillboard.shader, and it gave me the same error. Typically it would import fine, then the included shader would override the default built-in shader. Strangely this is NOT a problem for the SpeedTree.shader, only the billboard one.

Also, if I just use the built-in SpeedTreeBillboard.shader, not trying to override it, there's no error and it works as expected.

I filed bug report #894256.

Also, as side-question, I noticed the shader has tagged "noinstancing". Does that mean SpeedTree billboards won't be instanced? There's still the checkbox on the material to enable instancing. Still trying to get a handle on how instancing works.