Is it disadvantageous for the applicant to disclose that information?
It can be, yes. As the saying goes, "the first person to give a number loses". If you're currently on a low wage for whatever reason, disclosing that may set you up for receiving a low offer, if you make it that far. You can always negotiate upwards towards market rate, but why start yourself out in a lower position than absolutely necessary?
Conversely, if you're on a high salary and you disclose that, you might be dismissed as "too expensive" even if you'd be willing to accept a paycut for a job at the new company. In this case, you probably won't even get an offer or followup, so you also lose the opportunity to negotiate downward to something the employer can afford if you put a scarily high number.
And is it a private and legal matter that the applicant might be sued
upon if discovered?
In most Western locales, your salary cannot be declared private and confidential, either because laws are on the books which protect the open discussion of salaries or because the required burden of proof in a legal case is untenably high (for instance, it's not generally sufficient to prove that the salary was declared confidential and then disclosed; it's generally also necessary to prove that the salary was declared confidential to protect a legitimate business interest, that the business has suffered some tangible loss as a direct consequence of the salary being disclosed, and that in monetary terms the tangible loss is exactly $x). Some employers will try to do so, but such terms are not generally enforceable. Though as always, you should check with a local lawyer to determine what applies in your specific locale.
In practical terms, you do not need to worry about disclosing your current salary in an interview. However, you should carefully consider whether doing so is really in your best interest (for the reasons noted above).
How should you respond to the question "How much were you paid with
your previous employer?"?
I'd suggest something along the lines of "Unfortunately details of my previous compensation are considered confidential, however I can say that my compensation was in-line with the market rate for my industry and locale, and that I'd be willing to accept a comparable offer".
That avoids naming a specific number (albeit for a spurious reason), and sets expectations at 'market rate' compensation, which is a fair starting point all around.