EUREKA!
Discovered the solution by accident.
To turn the histogram back on in the viewfinder in the Nikon Z8 (if you are using firmware version 2.01), do this:
1) Go into the "Custom Settings Menu".
2) Scroll to item d9 ("View mode (Photo Lv).
3) Tap it (or click the "OK" button on the back of your camera).
4) On the next screen, tap "Show effects of settings".
5) On the next screen, tap "Always".
You should now see the histogram appear in the lower-right corner of your view finder.
I also discovered that now with version 2.01 of the firmware there have been two new items added to the "d Shooting/display" menu. So if you're looking through your original manual to find info on the settings in that section, the numbering will be off.
The two new items are:
• d5 is now "Exposure delay mode"
• d18 is now "Half-press to cancel zoom (MF)
I hope some of you find this information helpful.