Menu Bar Display icon has disappeared from the macOS Menu Bar The Display icon has disappeared from my Menu Bar and I cannot figure out how to get it back.
The ability to add it to the toolbar has disappeared from System Preferences &gt; Displays

I found an excellent article that contained instructions on how to add it back in this scenario:

For one reason or another, some menulet options remain hidden in
System Preferences by default. Interested users can find the full
collection in the System folder: Open a Finder window, from the menu
bar select Go -&gt; Go to Folder..., and then navigate to
/System/Library/CoreServices/Menu Extras.
Some of the extras in this folder are pretty obscure, but one or two
might come in handy – like Eject if you use an optical drive, or Ink
if you connect a graphics tablet to your Mac, for example. Just
double-click any that look useful and they'll be added to the menu
bar. They can be removed just as easily using the Command (⌘) key
method described above.

When I try this method, the Display icon does indeed show up on the Menu Bar again...
BEFORE:

AFTER:

However, when I try to click it, it immediately disappears.

What am I missing?
I'm running Catalina (10.15.6) on a 2015 Macbook Pro.
Looks like you have a lot of third-party widgets in your menubar. That's not inherently bad, but I'd suggest rebooting into safe mode and attempting to reproduce the behavior there.
Whether you can or cannot reproduce it, there's a chance this will resolve the behavior when you reboot again into normal mode. It's magical voodoo when this happens, but it has worked for me on similarly mysterious problems in the past.
If the problem disappears in safe mode and returns when you come back to regular mode, try disabling your third-party menubar widgets one at a time.