The ones that are always sweetened
Hibiscus, everywhere it is drunk — Egypt, Sudan, Mexico, Jamaica, Nigeria. Egyptian helba. Greek mountain tea, with honey. Sea buckthorn, barberry and sour cherry. Olive leaf and sarsaparilla. In each case the sweetener is part of the recipe rather than an indulgence added to it.
What sugar is doing
Not masking a fault. In a sharply sour drink the sweetness is what makes the acidity legible rather than merely aggressive, which is the same reason lemonade is not just lemon juice. A tradition that has sweetened something for generations has usually settled on a proportion worth starting from.
Botanical sweeteners
Liquorice root sweetens whatever it is simmered with and brings a strong anise flavour, so it is a flavour decision as much as a sweetening one. Stevia leaf is intensely sweet with a green bitter tail that the refined extract removes; one leaf is a cup and two taste of the plant.
Where sweetening is wrong
Anything already sweet. Beetroot, carob, linden and corn tea are sweet as brewed and sugar makes them cloying. So does sweetening a drink whose bitterness is the point — kuding’s sweet aftertaste only arrives because the bitterness came first.