The switch that completely isolated power from your pc. Luckily that still exists at the wall .... for now.
Phones are another story.
Not if your PC is a laptop, and has a battery. You can tell it to go to sleep, and, sometime later, Windows 10 will wake it up and sneak up an update on you (with a complimentary restart included).
I miss buttons in general.
My appartment has shared laundry facilities. It finally died, but I still miss the previous laundry machine, which had these big physical buttons that had this satisfying clock when you pushed them in.
The more recent but quaint days of desktop programs where clicking on [x] would actually close the program.