TLS 1.3 will require OpenSSL 1.1.x, which is only currently available on pfSense 2.5.0 development snapshots. Though it does look like net/haproxy-devel is at 2.0-dev2 on the branch used for pfSense 2.5.0 development, but the pfSense haproxy-devel package doesn't use it (yet).
I'm not aware of any plans to switch that over yet, but it's probably just a matter of time.