Also miss it. I took a couple of years to warm up to it. Then when started to I really really like it, it went downwards.
I‘ve actually stopped using Unity a few month before Canonical abandoned it because with all the Gnome applications moving into a completly different direction it didn‘t feel consistent at all anymore.
You can do
su
to change the user in the current shell. Afaik it just defaults to root if no user ist specified. Everytime you runsu
you actually dosu root
That said I always thought that it stands for switch user so intereresting to know that it‘s substitute.