How to overcome "Target Display Mode" from preventing sleep after disconnecting?
sudo killall dpd works quite well! It seems like it is not working, because ps -ax still lists dpd as a running process. But, when calling pmset, the not on "prevented by dpd" is missing:
pmset -g | grep sleep
hibernatefile /var/vm/sleepimage
networkoversleep 0
disksleep 10
sleep 10 (sleep prevented by sharingd, backupd)
displaysleep 10
dpd seems to be restarted right after killing it, which does make sense to always allow Target Display Mode to be used. But it looks confusing at first.