It can protect whatever the contract says. Usually, severance packages include a nondisparagement clause. I highly recommend requesting that they amend it to say "mutual nondisparagement" and re-word the terms to apply to both the employer and employee. That way you get paid to shut up, but they are also forced to abstain from making potentially disparaging statements about you. It's a good ask.

It’s a contractual agreement to not air dirty laundry in exchange for money. If you want to bitch about the company publicly, don’t accept a payment not to.

Unfortunately, this is not true. An NDA is not a free pass to being able to censor whatever you want, even if the person signing the NDA took a fee, or received payment.

"NDAs do not prohibit people from reporting suspected corrupt conduct to an appropriate authority. The Crime and Corruption Act 2001 and the Public Interest Disclosure Act 2010 provide safeguards that allow people who have signed an NDA to report suspected wrongdoing, including corrupt conduct, maladministration and the misuse of public resources.

Under no circumstances do they oblige people who have signed them to maintain secrecy about suspected wrongdoing. You can still report suspected wrongdoing despite signing an NDA."

[1] https://www.ccc.qld.gov.au/sites/default/files/Docs/Publicat...

I don't know that I'd consider Glassdoor "an appropriate authority" to be reporting corrupt or illegal conduct to.