I definitely don't remember ever seeing or changing that setting, but it is 1000 times better with it off. Guess it's worth looking through an app's settings every once in a while, no matter how long you've been using it, just to see if there's anything new (or maybe forgotten) that would improve things for you.
When typing code with ```, Enter should not send the message. With this checked use ShiftEnter to send.
Might be an easy fix :)