Floating Screenshot

When selecting the Show Floating Thumbnail option in the Screen Shot options menu, you must actually take a screen shot of something before this setting is written as a property into com.apple.screencapture. Selecting this option, and then pressing escape to dismiss the interface fails to achieve the preceding sentence.

One must perform the first paragraph before the show-thumbnail property can be disabled, or re-enabled, because if it does not exist in com.apple.screencapture, you cannot add it from the Terminal.

Inspect:

defaults read com.apple.screencapture show-thumbnail

Enable:

defaults write com.apple.screencapture show-thumbnail -bool TRUE

Disable:

defaults write com.apple.screencapture show-thumbnail -bool FALSE

One should not perform Terminal commands where there is no clear means to undo them, or avoid them altogether.

Do you mean the "floating thumbnail" that appears in the corner of your screen for a few seconds?

Press Command-Shift-5, then click on Options and check "Show Floating Thumbnail".

Use your trusty Time Machine Backup  from before the change.