After upgrade to Mojave, a long press of any letter does bring up the accented character menu but not anymore on the Touch Bar (Mac Book Pro 15-inch, 2018 with US keyboard).
Do you have an idea to to get them back on the touchbar ?
There is a possible reason why this may be:
You've installed the third-party product Webroot SecureAnywhere.
Webroot SecureAnywhere is made by Webroot Inc. When this software is installed on your Mac using its default settings, you might notice these symptoms:
Text that you dictate doesn't appear as expected. You don't see any accented characters when you hold down a letter key. You can't select items such as emoji in the Touch Bar of your 2016 MacBook Pro.
Follow these steps to fix the issue:
Click the Webroot SecureAnywhere icon in the menu bar. Choose Pause Secure Keyboard Entry.