What is a retention policy?
Retention policies are created by a company or organization and manage how long data is stored in the Microsoft apps you use. In Teams, these policies dictate how long messages show in a chat or channel and even how long a file is stored in Teams, OneDrive, and SharePoint.
Every organization can set different lengths of data storage, so to figure out how long your org's retention policy is, reach out to your IT admin.
If your organization—or an organization that you interact with via Teams—sets an expiration date for messages, a system message will replace the ones deleted by an org’s retention policy—including messages that you've pinned or saved. If the original post in a channel conversation expires, only the post will be deleted. All replies that are still within the retention period will remain.
When you have one-on-one or group chats with people outside your org, their retention policies can result in deleting messages from the Teams app. Messages are deleted for all members of the conversation. To learn more, see Messages in the Teams and Yammer apps are unexpectedly deleted by retention policies.
Note: Messages deleted by a retention policy can’t be recovered.
Files that are shared via a chat or channel message may have their own retention policy with a separate expiration date from the message. In those cases, the message and file preview may still show in Teams, but if you attempt to open the file, you’ll receive an error message that says, “File not found”.
If you’d like to learn how organizations create their retention policies, you can go to this technical document for IT admins: Create and configure retention policies
How do I delete messages before the expiration date?
If you want to delete a message before the retention policy does, simply hover over the message and select More options 
Note: You can only delete the messages you’ve sent so that you and others in Teams no longer see these messages. However, an organization might have set up a retention policy to retain a copy for compliance reasons.
