I think you missed the point. The point was not that github stopped using haproxy, but that it no longer needs to use it.
Assuming haproxy can work with unix sockets, github could configure nginx/unicorn to use it, but with unicorn its current work load does not require additional load balancing acrobatics beyond what the unix socket does.
Similarly, many rails sites use thin/nginx or mongrel/nginx and do not even have a workload that necessitates haproxy.
Plus, haproxy adds another layer of complexity, configuration, and management, which is nice to avoid if you can.
Assuming haproxy can work with unix sockets, github could configure nginx/unicorn to use it, but with unicorn its current work load does not require additional load balancing acrobatics beyond what the unix socket does.
Similarly, many rails sites use thin/nginx or mongrel/nginx and do not even have a workload that necessitates haproxy.
Plus, haproxy adds another layer of complexity, configuration, and management, which is nice to avoid if you can.