I've had companies asked for tax documents prior to giving me a formal offer. They didn't specify the purpose ("it's our process") but I suspect that it was to make sure the current salary I claimed was genuine.

I told them to f- off.

I've done the same, on one occasion I still got the offer. The only time I've given tax info is when I was working for an industry that mandated it (due to working on critical infrastructure and DOE regulatory requirements).

Better yet telling them what salary you want and not disclose your previous salary. Don't start you work relationship with a lie.

That sounds nice, but it seems like most if not all work relationships are based on at least dishonesty through omission if not outright lies.

Employers rarely give a full and true picture of what it's like to work somewhere. It's mostly "everything's great" and "all the people are great" and so on. You might get a bland "we need to work on X" if you press them, but they're very unlikely to be direct about the day-to-day drudgery, dysfunction, etc.

Due to the asymmetrical power relationship of employers/employees, I can't see a real ethical problem with lying in response to a question they shouldn't be asking in the first place.

But then you lose the chance to negotiate up. Any number you state can be interpreted as the highest you will get offered. In salary negotiation, make it so the employer presents the first offer, and then negotiate up.

Give them some range and do due diligence, the market figures are usually known

I've never found a range to work. They will immediately go to the bottom of your range or even below. So, the range you give better start at the top of your ideal range. Or, more preferably, don't name a number.

Could be considered fraud. This law removes a lot of concern by providing clarification. Some employers ask for pay stubs, but I agree with you, making up a number should have been OK on the grounds of privacy. Even creating a fake pay stub for this seems reasonably moral if the sole purpose is to set the salary expectation at a new job. Current salary and value aren't necessarily correlated and hires should be judged on their skills and value.

Not fraud. Employer's aren't allowed to give it out either. So you really have no incentive not to fib

Fraud - wrongful or criminal deception intended to result in financial or personal gain.

lying about your salary is not fraud. it has no impact on your performance or ability to do the work. your employer has no right to know your previous salary and your previous salary has no impact on what you're worth at a new position.

Don't some of the background check companies actually verify previous salaries?