How do I remove the new apps sidebar from Outlook 365?

For the moment you can still turn it off as per steps here: Customize the Navigation Bar - Microsoft Support

1. Select File > Options.
2. Select Advanced, and under Outlook panes, clear Show App in Outlook.  
3. Close and then restart Outlook.

 Uncheck this box

Update 10/04/2025

This still works for the current Microsoft 365 version.