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Presumably it's a function of your tenure at the company.

And your willingness to go quietly.

^ always. No bad mouthing the company and no poaching for at least 1 year is a standard clause.

Serious question, how can they enforce this?

You sign a contract. If you violate it, they sue you to get your severance back.

By burying you in attorney's fees until you crumble.

Severance packages always come with strings attached.