As ore and more activities occur on Microsoft Teams, the Office 365 Compliance team is now providing ability to put on Legal Hold Teams content (including private channels) on Legal Hold when you need to prevent content tampering.
To do so,, logon to your Office 365 Compliance portal (either https://protection.office.com/ or https://compliance.microsoft.com/ [the new Compliance portal]) and reach out the eDiscovery blade to create your Legal Hold case
Then create your new case and then edit it by opening the case to define the location(s) to be put on Hold
There you go 