Based on documentation: https://debezium.io/documentation/reference/configuration/outbox-event-router.html#_using_avro_as_the_payload_format

the outbox EventRouter transform can be used to pass along a payload of bytes (as suggested in the documentation, an avro message that has already been serialized).

However, the EventRouter is hardcoded to expect a String type for the event key. This prevents the use case of Avro encoded message and key, where both the message and key are serialized before writing to the outbox.

The hardcoded type is here: https://github.com/debezium/debezium/blob/de424d4980ddd8b2a2e75def90189b4e27f7c605/debezium-core/src/main/java/io/debezium/transforms/outbox/EventRouter.java#L169