Unless I'm mistaken, it's a manually filled in field, so apart from checking if files have gone through this stream one day via search criteria, I don't see.
Since you can't delete states that files would have gone through, I don't think you can delete a stream if these files are still present in the database (never tested this case).
The "IMPOSSIBLE" argument may be put to prevent a file from ending up in the workflow.
Yes, absolutely , it's a field that you fill in manually (with the conditions attached to the image).
But I don't remember when creating this flow why I put it "IMPOSSIBLE".
I think it was built and not operated afterwards. I would like to find a way to find out, whether or not this stream is active or not and delete it if it doesn't have it. Following your reasoning, I can try in the flow to delete the initial state, this will give me an indication on that.