Visual Studio 2019 16.4 Preview 2 adds the vertical tabs layout option to the core product:
Tabs Studio continues to work as usual if you keep the tab layout option set to the default Top value, but I’d like to use this opportunity to improve vertical tabs usage experience. More specifically, when vertical tabs are activated in VS 2019, Tabs Studio reuses this area to show tabs, instead of showing them in a separate tool window:
If you want to try it yourself, install Visual Studio Preview.
Enable vertical tabs in Visual Studio options:
And install Tabs Studio v4.6.2.
Tabs Studio Blog (organizing Visual Studio document tabs) 