The 3.4.0-RC2 version of Postfix appears to have dropped support for logging via TCP Unix sockets. As recently as 3.3.0 Postfix used a TCP Unix socket to connect to syslog. This is obliquely referenced in the release notes:
[Incompat 20190126] This introduces a new master.cf service 'postlog' with type 'unix-dgram' that is used by the new postlogd(8) daemon. Before backing out to an older Postfix version, edit the master.cf file and remove the postlog entry. I was able to work around this by logging to stdout and piping to the logger command, but perhaps it's worth a separate callout that the old logging behavior is no longer supported. Perhaps there's a way to do this in 3.4.0-RC2 that I've missed? Best, -Lex
