Without any database activity, I'm seeing this message in the logs:
connect_1 | 2020-03-02 11:42:30,047 WARN Postgres|dbserver1|postgres-connector-task Received 10001 events which were all filtered out, so no offset could be committed. This prevents the replication slot from acknowledging the processed WAL offsets, causing a growing backlog of non-removeable WAL segments on the database server. Consider to either adjust your filter configuration or enable heartbeat events (via the heartbeat.interval.ms option) to avoid this situation. [io.debezium.connector.postgresql.PostgresStreamingChangeEventSource]
The reason being that readPending() yields null to the consumer if there's no pending change. I.e. null is a regularly expected value here, but it triggers that message.
Also, and more importantly, this circumvents the throttling implementation, as readPending() doesn't return false in this case.