Your future salary expectations likely have a basis in your current salary. Prospective employers know this. Prospective employees know this. If expectations are stated, with that understanding, then your current salary should be irrelevant. If you're having trouble signaling your value to a prospective employer, then your current (good) salary, seems like an easy way to communicate your value, but it's flawed. Salary is far too reductive of a valuation of yourself. If your value is so difficult to communicate, then perhaps a little self-reflection would help you to pinpoint your strengths to better showcase them.
Further, there are few positive reasons for a prospective employer to seek that number. While it offers them market research into salaries to be competitive, it mostly lets them optimize their offer for the bare minimum to bring you on board. It lets them offer in such a way that you'll compromise your expectations for something slightly better salary-wise and different. I liken it to purchasing a vehicle from a dealer and knowing their margins before hand. This is an oversimplification, and they can say 'no' to a low margin, but it's far easier to persuade them to say 'yes' when you still know they're getting something positive out of it.