If you have a boss/employer threaten you will some bullshit claim of slander or libel please know this.
The perfect defense to slander is the truth. If the statements are factual and not false there is not a damn thing they can do to you legally.
NOW , if you signed an NDA, you made a contract agreement not to say anything in exchange for money.
However, in some cases if you signed under duress or threat of retaliation, it can be thrown out.
Obviously every situation differs but you all have more power than you realize.
Best thing any of you can do for yourself is make a hobby out of reading the labor laws, code of conduct , anything that gives you knowledge about your rights as a worker.
When you gain that knowledge, they will know not to fuck around with you. The WILL look at you differently even if they won't verbally acknowledge it.