I believe you have a right to "freeze" your work number profile so that they cannot share your past employment and salary information, at least if you live in California. They warn you that other jobs may not be able to verify your employment but I have never had an issue since asking for and being granted a hold on selling my information.

On a side note, never let past salaries dictate your salary negotiation. Always stand your ground, and sell your pricetag. Your past low salary or lack of successful negotiation, does not warrant them a discount. Negotiators always try to talk down your value, even when you're already paid high. You need to own your pricetag.

That is in the US, though.

Maybe in the UK too, but no in the EU,I think Sweden or Norway have total transparency on tax returns etc, though.

Or just say some percentage is a bonus or equity. And especially, ask them for what you want anyway.

I have guessed too low a couple times before, so now I simply go for current rate *1.5 and see if it sticks.

I've never had anyone ask me about my current salary in an interview, isn't that kind of illegal? Or surely at least considered poor form...

doesn't include equity though which is usually a large chunk of comp

The vast majority of people do not get equity as part of their compensation. So, yes, there are cases when this information is inaccurate or incomplete, but for most people, this type of data can artificially cap your earning ability.