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.
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.
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.