I just want to use this shortcut for one other application.
defaults write -g NSUserKeyEquivalents -dict-add "Special Characters..." nul
Or change the shortcut to something else in System Preferences:
Going to System Preferences and selecting Keyboard preferences, and then clicking on the Keyboard Shortcuts tab will show a list. If you select Keyboard & Text Input, the shortcut (Cmd + Option + T) should be there, and just follow the directions to change it.
In Mojave, selecting an Input Source as "Unicode Hex Input" in [ System Preferences > Keyboard > Input Sources ] does the trick.
Note that when you click the + to add this Input Source it appears way at the bottom under "Others".
Make sure to enable "Show input menu in menu bar" and select the new input method from the menu bar since it doesn't seem you can make this the default from the Control Panel.
Unfortunately, this is not a great solution for anyone needing a layout other than the basic English one. Some might find it suitable to just use the menu bar to switch to this input source when needed (ie. coding) and back to the native one otherwise.