The service running since 2020 that nothing depended on

A systemd unit enabled and running for five years, consuming a gigabyte, serving a feature that was removed in 2021.

$ systemctl list-units --type=service --state=running 
  | awk '{print $1}' | while read -r u; do
      pid=$(systemctl show -p MainPID --value "$u")
      [ "$pid" != 0 ] && ss -lntp 2>/dev/null | grep -q "pid=$pid" 
        || echo "$u: no listening socket"
    done

# which is a weak heuristic and found it anyway:
#   td-thumbnailer.service: no listening socket

Nothing alerts on a service that is working perfectly and is not needed, and the cost was a gigabyte of memory and a process that had been restarted by systemd nine hundred times over five years without anybody noticing the restart loop. Listing running units and asking what each is for is a half-hour exercise that belongs in the same quarterly pass as everything else here.