You and your employer must follow certain legal formalities for a severance agreement to be enforceable. You must ensure that the agreement is in writing and that your employer has signed it. Sometimes, you might also need a witness or have it notarized. Failing to adhere to these formalities can void the agreement.
Severance is never a requirement of any employer unless you have a signed employment agreement stating otherwise, or, it is a written policy of the company.
It makes no difference how long you've been with a company so yes, it's legal to lay off any and everyone without severance. The exceptions: a union agreement requiring severance, a personal contract calling for a severance. This is usually only for executives and ``key'' people.
Lack of Voluntary Consent: Under California law, a severance agreement can be considered valid and enforceable only if the parties entered into it voluntarily. If your consent was obtained through coercion, duress, or fraud, the agreement will be deemed invalid.
Employers are not legally required to offer severance during layoffs, but many choose to do so to maintain goodwill and ease the transition for their former employees. If you are offered a severance agreement, remember that you don't have to sign it right away.
To increase your chances of a successful negotiation, choose a reasonable counter-offer. Think about the resources your former employers can offer and what you can offer in return. Employers usually do not want to engage in a lengthy negotiation, so presenting a reasonable offer may encourage them to accept to move on.
You do not get severance if you quit. Nobody is automatically entitled to any severance legally, ever, unless you were hired under a contract such as a 1099 employee and you have severance written into your agreement. Standard W-2 employees usually do not get severance.