Does macOS Mojave support NightShift on external monitors?
Night Shift is working on external monitors in Mojave.

Sometimes Night Shift deactivates itself on the external display when you wake the system from sleep. (Maybe a Bug?)

Just reactivate it manually.


If Night Shift is off, go to the Night Shift tab in System
Preferences>Display and check the box labelled Turn on until
tomorrow.
If Night Shift is on, click on the Schedule menu in the Night shift
tab and choose  ‘Off.’
Use the color temperature slider in the same window to control the
degree to which colors change when Night Switch is turned on or off.