should it include in official github branch or any separate github link or else?

Mostly for doing animation - certain controller bones that doesn't affect to final result and yet it still exist on file. It could cause animation size unnecessarily bigger, and due to SMD it self have limited bone count, some bones might can left off from result - and I already have
few experience with this case.

To prevent this, we should add feature that exclude bones that we want.
I think to decide which bones that exclude from export, the bone group feature exist on blender might make it mostly comfortable.

title

OK, i was forget $bonemerge, I was stupid, thx.

I mean, not such an attachment,
Such like object that attaching on guns.

If the bone have vertex group that weighted, it detected.
but, i need to make bone that don't have vertex group for attachments.

I just make empty(no vertex group) bones.
in export, there's no problem but,
in compile, these bones not detected.
it means, bones cannot defined.

ah, i just forget one thing, My export error was "parent bone id" and your export error is other Xd

actually, it fixed, in new version,
if you using outdated version, just export it as fbx (or other extension)
and import it, and export it smd again.

Damn, I have same problem, i hope we can solve this problem ;((