Going back a few days:
Hmmm... I just noticed this: under "File", command-T is "New Tab", and under "History" it's "Reopen Last Closed Tab" (which command-Z used to do).
Can't help with your Safari troubles but in case you're not aware, command-z is Undo.
PS: Pendragon, I moved your post to a separate thread in the Mac OS X Applications forum:
SuperDuper slows computer in Sierra