Microsoft rolls out a new feature which allows to manage the discovery of private Teams.

Source: Microsoft 365 Roadmap (August 2019)

Conclusion, opinion and summary

To manage the discovery of private Teams allows that Teams are listed and searchable like public Teams. However, an user cannot directly join a Team he/she need to get approved by a Team owner in order to join.

You’ll be able to manage this per-Team either via PowerShell or in the Teams GUI.

Addition resources