Then she would get no money either.

I don't think that's true in most situations. I think most companies would still pay a severance out of a general effort to smooth things over, and some sense of social loyalties.

And some aspects of the agreement would be fine. No spilling actual business relevant secrets, nothing that would count as libel or slander, etc.

Nope, they'd have security show you the door and maybe threaten to sue you into the ground if you talk. They choose mkney because money is easier, but if you take this option away, you just get the stick and no carrot.

Part of smoothing things over is implicit of explicit understanding that employee also smooths things over that is shut up after receiving the bribe.

Companies are people, many people have positive feelings towards co-workers and people like them

True but not the ones who will be firing you. At least not in big corp, it would be some HR worker who doesn't know you from Adam and couldn't care less about you.

They always have the manager, and the director and possibly VP will be in the loop and know what going on.

The immediate manager will be informed, but they can do nothing about it or in any way change the process or the result.

But then Facebook would have had no incentive to offer the agreement to her. And she must have been very happy with what she got in the deal, given she gave up the option of talking about all the apparently horrible misdeeds she witnessed while they were still fresh, rather than a decade later.

Plenty of companies have nonenforcable clauses in their contracts. Either by momentum or stupidity. That's despite the inclusion of such conditions can often void a contract in its entirety.

Is there an incentive to have unenforceable clauses? Yes, of course. The same way one might be incentivized to say they will harm you if you do not do what they want even if they have no intention of harming you.

It's called "a bluff". Play some poker, you'll see how effective this strategy really is.