The French do.You don't braze plumbing fittings.
If you were to braze the joint with a copper/phosphorus alloy brazing rod, you don't need a flux.
You don't need an active flux, like Powerflow and the like, that makes it unnecessary to clean the joint surfaces, because they produces acidic compounds that clean the surfaces. The flux residues also get into the circulating water, making it acidic and promoting bi-mettalic/galvanic corrosion and turning the insides of the radiators into black magnetite sludge.
The French do not have problems with black sludge and they do not have a power-flushing industry.
Gas capilliary fittings are soft soldered, but gas fitters aren't meant to use active fluxes.
Refrigerant fittings are brazed, but the fittings are a different design.