Then this guide is for you.
After you uncheck this option, the members of your team wont be able to create new private channels.
The option remains visible on the app interface even if you lock it.
By the way, you better roll out the Azure AD Module to do that.
More specifically, you need the AzureADPreview.
To do this, launch PowerShell with admin rights and run theInstall-module AzureADPreviewcommand.
Using this method also brings another advantage.
When Office 365 users create an Outlook group, the system no longer creates a Team associated with it.
