Trying to connect to Oracle 19c using client 18c, here are the versions:

Client: 

Release 18.0.0.0.0 - Production on Thu Sep 10 08:55:08 2020

Version 18.3.0.0.0

Server:

Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production

Version 19.3.0.0.0

Debezium will not start and throws error:

[2020-09-10 08:41:57,785] INFO Creating thread debezium-oracleconnector-orcldb-change-event-source-coordinator (io.debezium.util.Threads:287)

[2020-09-10 08:41:57,800] INFO WorkerSourceTask{id=hr-connector-0} Source task finished initialization and start (org.apache.kafka.connect.runtime.WorkerSourceTask:209)

[2020-09-10 08:41:57,803] INFO Metrics registered (io.debezium.pipeline.ChangeEventSourceCoordinator:89)

[2020-09-10 08:41:57,803] INFO Context created (io.debezium.pipeline.ChangeEventSourceCoordinator:92)

[2020-09-10 08:42:00,119] INFO Snapshot step 1 - Preparing (io.debezium.relational.RelationalSnapshotChangeEventSource:95)

[2020-09-10 08:42:00,120] INFO Snapshot step 2 - Determining captured tables (io.debezium.relational.RelationalSnapshotChangeEventSource:106)

[2020-09-10 08:42:02,689] INFO Snapshot step 3 - Locking captured tables (io.debezium.relational.RelationalSnapshotChangeEventSource:113)

[2020-09-10 08:42:03,172] INFO Snapshot - Final stage (io.debezium.pipeline.source.AbstractSnapshotChangeEventSource:79)

[2020-09-10 08:42:03,181] ERROR Producer failure (io.debezium.pipeline.ErrorHandler:31)

java.lang.RuntimeException: java.sql.SQLSyntaxErrorException: ORA-01738: missing IN keyword