Zookeeper has implemented dynamic reconfiguration since circa 2013-2015ish, but it required support for the feature in clients, as well as servers. That said, implementing an autoscaler that manages that as well was not trivial, in my experience.
This is only half true. Yes, the dynamic reconfiguration features has been in trunk for a while but ZooKeeper 3.5 which is the first GA release to include this has only been released 4-8 weeks ago.