I’m not sure about Beersmith or other recipe apps, but Brewer’s Friend gives you an estimate of calories and grams of carbohydrates in the recipe builder.
The March/April '24 Zymurgy that published last week has a piece on Low-Carb beers, which includes information on determining the level of carbs. It’s called “The Art and Science of Low-Carb Beers” by Samuel Loader (page 44).
I am looking forward to receiving the issue and reading the article. A couple of years ago I went on a low carb diet and lost 40 lbs. For beer I drank mostly Michelob Ultra at 2.5 g of carbs per can. I got tired of drinking Michelob Ultra and went back to drinking normal beer. I haven’t gained any weight back, but I haven’t lost any either. I would love to be able to brew a 2.5 g carb/12 oz. version of some of my favorite beers.