The ssh client had no keep alive. And the sshd had no keep alive. There was not firewall and a long DHCP lease. So it all just worked.

But because of firewalls aggressivly closing xonnections, these days we tend to need keep alives and so I don't this this would work today.

    alias my-server='ssh server-hostname -t "tmux new-session -s user || tmux attach-session -t user"'

You could also tell it to try reconnecting every second and bind it to something easy to type like 'kj' (too bad df is already taken)

    alias kj='while true; do my-server; sleep 1; done'