When streaming data via Kafka to a realtime table, does it have to be 1 record per message or is there a way to put multiple records in a single message?

one record per msg only

I guess you could use a JSON column to effectively store multiple records per message.

You can also use flatten udf to convert one message into multiple records

The flattening "one record into many" documentation states this isn't natively supported yet -- is that still correct? I'd have to write my own decoder/reader?