It's true for protocol version 1 that only the committed data is sent on the replication connection.

In protocol version 2 (introduced in postgres 14) large in-progress transactions can appear in the new "Stream" messages sent on the replication connection: StreamStart, StreamStop, StreamAbort, StreamCommit, etc. In the case of large in-progress transactions uncommitted data might end up in the replication connection after a StreamStart message. But you would also receive a StreamCommit or StreamAbort message to tell you what happened to that transaction.

I've not worked out what qualifies as a "large" transaction though. But it is _possible_ to get uncommitted data in the replication connection, although unlikely.

https://www.postgresql.org/docs/15/protocol-logicalrep-messa...