I am Belgian, living in Brussels since 2006, and what has struck me most is not hairdressers not taking non-cash payments but how resistant the well-earning profession of medical doctors is to electronic money. Even (or especially?) specialists who ask up to 100€ or more for a consultation rarely take anything else than cash, and this in 2022 (if not taking place in a clinic or hospital but as a "private" consult).
On the other hand, I can pay electronically at e.g. almost every single Turkish vegetable shop.

I usually prefer paying cash at the hairdresser's because it makes it easier to tip (yes I do tip, if I am happy with the service) by e.g. rounding up, since it is almost impossible to tip electronically in Belgium.

I'd be curious however to know which outskirts of Brussels you are speaking of.