ClickHouse Keeper is a ZooKeeper alternative.
The network protocol used by clients to talk to ClickHouse Keeper is compatible with the ZooKeeper protocol. This means you can use any ZooKeeper client to talk to ClickHouse Keeper, and the ClickHouse keeper-client to talk to ZooKeeper servers.
The protocol used by ClickHouse Keeper nodes to talk to each other is not compatible with ZooKeeper. This means you cannot run mixed clusters with some nodes running ZooKeeper and others using ClickHouse Keeper.
ClickHouse developers guarantee that versions of Keeper within one year of each other can interoperate, but they provide no further guarantees than that. If you are upgrading an old (older than one year) CH Keeper cluster incrementally, it might be better to update multiple times so that you never jump more than 1 year at once.