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.
[UPDATE January 30, 2020] After some discussions with a colleague I decided the world needs a Plugin-based approach to the solution below. We talked through how it’d work before going about our day. After writing the info file, .module file, and prepping the folder structure I realized I never checked to see if this already existed.
I was pleasantly surprised to see that Sutharsan had already created a module that fits the bill! Extra Field is nearly identical in structure and implementation as what I’d planned to build. I just finished writing my first ExtraField\Display plugin and it works beautifully! I will leave my post below in tact, but I highly recommend stopping here and simply using Extra Field. Note that the README.txt file is worth reading, and there is an extra_field_example sub-module that explains things quite well.