When will the changes be coming?
Current state - Left navigation is on for anyone who is in a Stack Overflow team.
Late April - Finish core implementation, addressing early feedback, enable responsive design for Stack Overflow for Teams beta users.
May - Address bugs and critical feedback
Early-June - Sneak peak on MSE. Deploy to Stack Overflow and MSO.
Late June - Address bugs and critical feedback. Gather feedback on preliminary site designs for 5-10 existing communities with custom designs.
Begin deployment for all users on sites with custom themes. Plan is to do this roughly by site age, though we reserve the right to vary the order due to implementation concerns. Outline process and timeline for sites without custom themes to get one.
July and beyond - Design, community review and deploy site themes to additional sites.