Agreed but why is there a message specification for it describe in the documentation and it ask to client reply back if a particular *bit* is set.(1 means that the client should reply to this message as soon as possible, to avoid a timeout disconnect. 0 otherwise)
This is unrelated to TCP keepalive. I honestly don't know where the knob is to turn these on but the configuration variables you quoted earlier I am familiar with and they are not it. Perhaps someone else can chime in with how to enable the protocol level keepalive in replication.
Protocol-level keepalives are governed by "wal_sender_timeout"