Worker-versioning when can I kill old worker?

In my example I have 2 versions both workers are running simultaneously
And everything is working as expected the ExistingWorkflows keep being executed on old workers and all the new ones go to the new :smile:
Except for 1 thing I cant understand how can I know for sure that I can kill 1.0 workers?
Running this command:
temporal task-queue get-build-id-reachability --build-id=1.0
I got:
1.0 pizza-queue [ExistingWorkflows]
But there are no workflows waiting for queue 1 they are all complete
What am I missing is this a bug?
Is there another way to be sure I can kill worker 1.0?