I agree insofar as it is indeed an illegitimate question designed to give them an idea of how low they can go.
However, there's two things worth noting:
if salary is your main driver for a new job, you should have key figures in mind anyhow - there'd be no point negotiating down against your previous salary if you already have an ideal and a lower bound in mind. In fact, you can precisely state that you aren't happy with your previous salary and that's why you want a significantly better offer.
if salary isn't your main driver, you can still negotiate a harsher increase. Why not? After all, you can argue your other reasons for choosing them over other offers.
Questions (and answers) about your prior salary are absolutely in poor taste, but I would refrain from making that a hard pass on the company. Instead, you can very much use that to your advantage in negotiating.
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I agree insofar as it is indeed an illegitimate question designed to give them an idea of how low they can go.
However, there's two things worth noting:
Questions (and answers) about your prior salary are absolutely in poor taste, but I would refrain from making that a hard pass on the company. Instead, you can very much use that to your advantage in negotiating.