here's a couple articles regarding soda after WLS:
http://www.obesityhelp.com/articles/drinking-soda-after-wls-debunking-the-myth
https://www.livestrong.com/article/347882-carbonated-drinks-after-a-gastric-bypass/
The key is that you don't want to drink liquid calories. Also, you want to be cautious on how carbonation affects you. If you decide to drink diet soda, you want to make sure it doesn't cause excess gas that gives you discomfort. This is especially important during the healing process, up to 3 months. Also, that you are avoiding drinking and eating at the same time (which is the same for all liquids). Some programs allow carbonation, some do not.