Is it legal for my former employer to reveal my wages?

Who is your employer revealing your wages to? Generally an employer cannot reveal any information about you without your written permission.

There is nothing illegal about it. However, depending on the circumstances it might be a breach of ethics or privacy.

Depends on where you work and why, if you apply for a new job, you sign a release in your application to allow your new place of work ask for this,
But many jobs which are government jobs, or union jobs, the pay scale is posted. When I worked for the Government as a GS 12, anyone could just pull up the scale pay list and know what I made.