Sugar, and what it is doing besides sweetness
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In a fermented drink, sugar is not a flavor decision. It is the fuel supply for a reaction that continues after the sale.
Four jobs
Sweetness, obviously.
Body and mouthfeel, which is why a sugar-free version of a drink tastes thin as well as less sweet.
Preservation, at high concentrations, by lowering water activity, which is how a syrup keeps.
Fuel, for any live culture in the bottle.
Fuel is the one that matters legally
Residual fermentable sugar plus a live culture plus time produces alcohol and carbon dioxide.
Which connects directly to both failures in this property: the 0.5 percent line, and bottle pressure.
So in a live product the sugar level is a control parameter, not a recipe preference, and it belongs in the specification with a target and a tolerance.
Measured as Brix, which is quick, cheap and repeatable, and which is the number to watch through fermentation.
Sweetening without fermentable sugar
The usual answer for a live product that needs to taste sweet and stay under the line.
Non-fermentable sweeteners, stevia, monk fruit, erythritol and others, give sweetness the culture cannot eat.
Or stabilize first, by filtration or heat, and sweeten afterwards, which removes the live positioning.
Or ferment drier and accept a tarter product, which is what several established kombucha brands do and which the category's customers have largely accepted.
What it does to the label
Sugars and added sugars are declared, and the added-sugar figure is the one customers read.
A fermented product is an interesting case: sugar added and then consumed by the culture is not all present in the finished drink, and what is declared should reflect the product as sold rather than as made.
Which is a measurement question rather than a bookkeeping one, and it is worth getting the finished product analyzed rather than calculating it from the recipe.