With the NiFi 1.x baseline, zookeeper is a fundamental part of the NiFi cluster. It is used to automatically determine the cluster coordinator as well as communication within the cluster.

With the NiFi 0.x baseline, the cluster has an cluster manager which serves a similar role and did not use zookeeper at all. So, if you do not want to use zookeeper, you will have to use a 0.x baseline version of NiFi.