Reading the documentation: https://docs.couchdb.org/en/stable/cluster/index.html
As of CouchDB 2.0.0, CouchDB can be run in two different modes of operation:
Standalone: In this mode, CouchDB’s clustering is unavailable. CouchDB’s HTTP-based replication with other CouchDB installations remains available.
Cluster: A cluster of CouchDB installations internally replicate with each other via optimized network connections. This is intended to be used with servers that are in the same data center. This allows for database sharding to improve performance.
I am wondering if when it's running in Cluster mode, a client can still offline sync documents it is interested in?
Yes, nothing changes with respect to the replication API.