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If you have kitchen units you could put up a chipboard wall (or even MDF) and instead of having battens (not sure if right word?) use the cabinets. Then a small gap and something like dynamat on the existing wall. Like that there would be no bridging.
Decoupling and two heavy materials should do quite some work, but sound from stairs of course also travels through ceilings and floors.
You joke, but I’d be seriously considering guerrilla / gorilla gluing some rubber treads onto the stairs. Safer for everyone, quieter for you. Also a cheap experiment, and it’s not like you’d actually get in trouble for it. If I lived in the block I’d just assume the maintenance people had been told to comply with some new reg.