You can't add a contingency to a payment retroactively. It sounds like these are exit agreements, not employment agreements.

If it was "we'll give you shares/cash if you don't say anything bad about us", that's normal, kind of standard fare for exit agreements, it's why severance packages exist.

But if it is "we'll take away the shares that you already earned as part of your regular employment compensation unless you agree to not say anything bad about us", that's extortion.