Though I never thought the joining and revocation was all that difficult anyway. I just have 2 central servers which carry every key, and the others just need to have the server keys. Everything else gets distributed automatically. So keys for clients you can keep on your servers only. You don't have to delete it from all of your clients!
I know tinc doesn't really have a client/server concept but I consider clients those devices that aren't publicly reachable (behind NAT) and not used in a ConnectTo statement.
And yeah the centralised VL1 "Planet" server in ZeroTier also bothers me. I know it plays no part in the actual access rights to the network and it can't see the traffic content, but still. I just want to run it myself.