You can test haproxy v2 protocol support in postfix-3.5-20200105-nonprod (http://ftp.porcupine.org/mirrors/postfix-release/index.html). I have done all the testing that I can do. It would be great is someone can test it against some real haproxy client.
Haproxy v2 protocol support is limited to TCP over IPv4 and TCP
over IPv6. It supports non-proxied connections (typically used for
heartbeats).
This will be part of the Postfix 3.5 stable release early this year.
Wietse
