How can I achieve taxonomy workflow (allowed next states of taxonomy terms)?
I don't think there's a ready made solution for this. The maintainer of workbench_moderation has said that a goal is to collaborate with other workflow modules and made one module to rule them all, and one of the primary goals of that initiative is "workflows for non-node entities" but this is still a long way off and doesn't seem to be very far past the planning stages at this point.

So I think the closest you can get currently is the taxonomy_revision module but that just adds revisions and doesn't help in automating a workflow out of them, so you'll have to do some custom development on top of that.