the old direct-to-master TCP replication protocol has been deprecated since Synapse 1.18.0 (nearly a year and a half, as of this writing).

We still have a bunch of code to support it (DirectTcpReplicationClientFactory etc), and it's still mentioned in various bits of documentation (in particular https://matrix-org.github.io/synapse/develop/tcp_replication.html needs an update). Having multiple ways to do it is confusing and hard to maintain, so we should remove it properly.