Based on comment:
https://github.com/kamailio/kamailio/issues/171#issuecomment-418175840
I gave event_route [tcp:closed] a try:
event_route [tcp:closed] { # Handle TCP connection close xlog("L_INFO", "$proto connection closed ($conid)\n"); }
Nothing comes to syslog when I quit my baresip UA that according to wireshark closes its TCP connection after un-registration (fin - fin - ack).
I have not set closed_event param, because according to README, it defaults to 1.