After you’ve entered the deck overview screen (by clicking on a deck in the main screen), there is a button “description” at the bottom (next to “options” and “custom study”). When you click on it, the description window opens. The window contains this checkbox.
See ‘other changes’ in Changes in 2.1.40 to 49 - Changes
It allows the description to appear under the “Congratulations!” message, in the deck overview screen. Otherwise you can only see the description when clicking it’s button again.