I’m not sure about OP, but I have the same problem with v2.0.3.
I destroyed and re-created each of the 3 CockroachDB servers, one by one, while allowing the new one (with a new IP address) to connect to the cluster before moving on to the next.
I was left with 3 dead nodes, so I followed your guide to decommission them.
Now I have 3 “decommissioned” nodes in the dashboard and no way to remove them since, as OP pointed out, they no longer exist so the “quit” command is unhelpful.
Also, I’m not sure if this is related, but it seems wrong. It says “decommissioned since” and gives the time they became dead, which was actually yesterday for 2 of them, even though I only decommissioned them a few minutes ago.