In the past I’ve been happy to rely on Display Suite to create what I call “Frontend Only” (DSField) fields (which I created via php). These fields appeared in the Manage Displays screens so that site admins can easily drag them around in the display as needed. As it turns out, you can achieve nearly the same result using a few built-in hooks in Drupal 8. The Display Suite DSField plugin has a few handy helpers like the ability to only show the field on specific display mode forms (e.g., show the field on Full content but not on Teaser). You can still control whether or not the display-only field renders if you use the core hooks, so we’ll be okay.
Before I show the code, let me give some background.