Depending on what type of license your customer owns, not renewing maintenance or subscriptions has different outcomes. If you are unsure of how your customer's product is licensed, please reach out to your Customer Renewal Manager.
When your customer's subscription expires, their access to Tableau will be suspended after the subscription expiration date. Specifically for Tableau Online, Tableau cannot guarantee you will have access to any content published, including any data input into the product.
Perpetual Licenses & Maintenance
If your customer's Tableau licenses are perpetual, they may continue to use the product in perpetuity after their maintenance expires. They can continue to use the last available version while their maintenance was active indefinitely; but access to product upgrades and technical support will not be available without a current maintenance contract. If your customer opts not to renew, please have them keep a copy of the Software as Tableau will not guarantee supporting the download in perpetuity.
For more details see the Tableau Subscription & Maintenance Policy.