There is plenty to be said that is bad about carbonated beverages, but I wanted to address this one in particular. Depending on the surgery, the stretchy part of of the stomach (the fundus) is removed (sleeve) or bypassed (bypass). Therefore it is not possible to stretch the stomach again.
However! Once the scar tissue has fully healed, the stomach that remains does regain some pliability. I would liken it to a garden hose in the winter versus the same hose in the summer. On a properly performed surgery, when people say that their stomach has stretched, most likely what has happened is that the pylorus (for sleeves) or the created opening to the intestine (bypass) has been weakened and/or stretched and food can move from the stomach or pouch into the intestine quicker allowing for more food to be consumed.
To make the myth worse is the fact that doctors/surgeons will put the above into "laymans terms" for their patients. Thus perpetuating the belief that you can stretch your sleeve by eating poorly or too much or drinking carbonated beverages.