I’ve been intending to disable this ever since I upgraded to macOS Mojave. Today I finally found the time.1

The first thing I had to do was go to System Preferences -> Keyboard -> Shortcuts -> Screenshots and check the checkbox that says Screenshot and recording options, which was disabled apparently by default for whatever reason. After that, you can actually do the Shift-Command (⌘)-5 combo and follow the tutorial in Tek Revenue. (Its tl;dr is to simply un-select the Show Floating Thumbnail value in the screenshot utility options dropdown.)

  1. I logged 64 hours on my computer last week. Been busy! ↩︎