Update: After feedback, Microsoft has decided not to roll out this change:
One to watch out for as an admin. Previously “Private” Microsoft Teams did not appear in the Add Team / Suggested Teams / Search here:
Starting March 23, 2018, private teams will be searchable in Microsoft Teams. I think this makes sense, as it makes them more discoverable and Teams sprawl is 100% real. If a user requests to join a private team, the team owner will receive a notification and can approve or deny the request directly in Teams.
However, it might be worth checking that there are no private teams in your tenant with names or descriptions that people do not want to be public. If there is a Team called “Secret Management Buyout of Contso”, now might be a good time to rename that Team
If you wish to hide a private team or group, Office 365 admins can use the Set-UnifiedGroup PowerShell cmdlet to hide specific groups or teams (in Outlook, Outlook Web App, and Microsoft Teams). Specifically, use the HiddenFromAddressListsEnabled parameter, which you can apply to individual groups.