Low carb? Probably not. But there is also an issue of gluten, and other proteins. A breadmaker in San Fransisco (?) claims that his sourdough wheat bread was tested, and found to have no measurable gluten. I would assume that the other proteins would be similarly eliminated.
Even though I am low carb, I would have no qualms with having the occasional binge of sourdough bread. If I could find it, and be certain that it was made correctly.
I can’t help but doubt the protein angle as well. Acid does break down proteins and sourdough has more lactic acid than other bread. But it’s protein — namely gluten — that gives bread its basic quality of elasticity which lets it form stretchy bubbles and rise and gives it proper chewiness. If sourdough didn’t still have lots of gluten it basically wouldn’t be bread anymore. But that’s not the case, so it still must have lots of gluten. Now there are gluten-free breads available but they’re either unappealingly crumbly or they have non-gluten protein added.
I guess when it comes to bread I sort of accept it for what it is and just enjoy it as an occasional food like you mentioned.