When will the Microsoft Teams Classic client stop working? Do users have to be on the new Teams by specific deadlines? Is it possible for an organization to stay on the Classic Team for an extra year or longer? (since we have the option not to include the new Teams client with Microsoft managed Upgrade via 365 apps)

For example, If I plan to set the Global (Org-Wide default) settings "Use new Teams Client" to "Not enabled." Since I want to control the roll-out via MECM using the "teamsbootstrapper" for more planned adoption rather than the new Teams client just rolling out to machines. 

I want to mitigate known issues; changes/updates can sometimes impact users' daily work life.

Also, when will the offline install of the bootstrapper be available? Any dates? This will help with bulk deployment a lot