Generic product keys found online may never expire and can be mistakenly used for production, but they are 100% illegal for production use. Scam websites often sell these "lifetime licenses" using public keys, which can lead to a non-compliant status and potential audit nightmares.
While largely phased out for new purchases during the 2012 lifecycle, certain Volume License agreements permitted a Server + Client Access License (CAL) model for Enterprise Edition. i--- Sql Server 2012 Enterprise Edition Product Key
It is important to note that while these keys are widely available online, they are not a substitute for a valid license. Using such keys for production environments without a legitimate license constitutes software piracy and carries significant legal and security risks. Generic product keys found online may never expire