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.
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.
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.