And during multiple sessions at Ignite 2018, representatives from the Outlook team were doing demos where they were running Outlook Pro Plus (2019) with the new left panel 'account switcher' feature clearly active and working, but it has yet to hit the Office Insider Release Channel.
WHEN is this going to happen? This is a way overdue feature, and one that's preventing half my clients from being willing to move to Outlook as their primary mail client.
The first part of this is rolling out now. If you are targeted release in Office 365 (not Proplus channels of similar name) and on monthly channel, then you can turn on the coming soon option top right in Outlook to see the single row task bar. Account switcher I've not seen appearing yet.