Microsoft is moving the navigation buttons at the bottom of the folder list to a vertical bar on the left of the folder list (just like in Outlook on the web). To address complaints that the bar makes the folder list too wide, Microsoft reduced the minimum size of the folder list by 40 pixels.

Why is Microsoft moving the buttons for Outlook modules? To make the "Outlook experience" more consistent with other Microsoft 365 products like Teams, Outlook on the web, and Office.com. They added links to the other Office apps, and to new Microsoft 365 apps, including To-Do.

Vote for and comment on this suggestion. Hopefully, they will add a permanent solution for people who want to remove it.
Navigation bar chnge (Microsoft feedback)

"Snooze" the change

Beginning with Outlook Version 2211, released to the Current Channel in late November 2022, you have the option to revert the navigation to the bottom, at least temporarily, or using Microsoft's phrasing "snooze it".

Go to File > Options > Advanced and remove the check from Show apps in Outlook.

You will need to restart Outlook and the navigation buttons will be on the bottom again.

The registry setting for this is

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences
DWORD: EnableAppsInOutlook
Value = 0 (Nav bar on the bottom) 

If you have Coming Soon

If you have a current version of Office, you will either have a "Coming soon" button on the top right that will switch between the bar on the left (Coming soon on) or on the bottom (Coming soon is off). When the coming soon button is missing or replaced by the "Try the New Outlook" switch, the vertical navigation bar is permanent. Sort of.

A temporary solution (it will no longer work when the function is no longer experimental) -
Open the registry editor and locate an entry named Microsoft.Office.Outlook.Hub.HubBar and change it to False.

Change Microsoft.Office.Outlook.Hub.HubBar value to false

1. Type regedit on the start menu, open Registry Editor when it comes up.
2. Browse to the keys listed above to find Microsoft.Office.Outlook.Hub.HubBar
   It should be in one of these two locations, depending on which Channel your software is on.
   HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\ExperimentConfigs\ExternalFeatureOverrides\outlook
   or
   HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\ExperimentEcs\Overrides
   
3. Right-click on Microsoft.Office.Outlook.Hub.HubBar and choose Modify.
   
4. Type false in the Value data field.
   
5. Close and restart Outlook for the key to take effect.

If you do not have an entry in either key, you will need to add a string value for Microsoft.Office.Outlook.Hub.HubBar to the registry.

Add Microsoft.Office.Outlook.Hub.HubBar to the registry

1. Type regedit on the start menu, open Registry Editor when it comes up.
2. Browse to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\ExperimentConfigs\ExternalFeatureOverrides\outlook
3. Select the Outlook folder to display the contents on the right
4. On the right side of the screen, right-click and select New > String Value.
   
5. Name the new value Microsoft.Office.Outlook.Hub.HubBar
6. Right-click on the new string and choose Modify.
   
7. Type false in the Value data field.
   
8. Close and restart Outlook for the key to take effect.

If you don't want to edit the registry, you can download and run this reg file:
Microsoft.Office.Outlook.Hub.HubBar registry key