According to MC264092:
In the Teams admin center you can find the “Update policies” tab. Here you can use the default “Global” policy or create and assign other policies, in order to enable/disable the Public Preview channel feature for Microsoft Teams clients.
What will change?
Starting from 23rd of July, a new policy called “AllowPublicPreview” will be enabled by default for all tenants! This means that any users on the Current Channel (Preview) update channel for Microsoft Office will automatically also be enabled for the Public Preview channel in Teams.
What do I need to do?
If you use the Current Channel (Preview) channel for Office and have the Teams Preview enabled, nothing will change. For users not in the Office Update channel mentioned above, or users in the channel but shouldn’t have the Teams Preview features, you need to create/alter the Teams update policys so it fits the new configurations.
You can work with the update policys from the Teams Admin Center (https://admin.teams.microsoft.com) or use PowerShell with the “CsTeamsUpdateManagementPolicy” cmdlet (Public preview in Microsoft Teams – Microsoft Teams | Microsoft Docs)