When an internet provider retires a plan it is no longer available to new customers.
However those who already had the plan are generally grandfathered in - meaning they get to keep their plan, features and pricing for as long as they keep paying the bill. The carrier does legally have the option to discontinue the plan or change terms at any time. And sometimes carriers are very clear that promotional plans are for a limited time only, and will not be grandfathered in after the time period is up.
Sometimes grandfathered in plans can be transferred to other customers, but this is carrier and plan dependent.