Microsoft has two "new" Outlooks, one for MacOS and one for Windows. The Windows version will replace Windows Mail & Calendar in late 2024. Microsoft is hoping to eventually replace classic Outlook with new Outlook, but that is years away. It's just not robust and lacks too many features.

While you cannot turn off the option to try the new Outlook in Windows mail, you can turn it off in classic Outlook for Windows.

If you want to disable the option "Try the new Outlook", you need to set this registry key:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\General
DWORD: HideNewOutlookToggle
Value: 1

reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Options\General" /v HideNewOutlookToggle /t REG_DWORD /d 00000001 /f

If you prefer to use .reg files, I have this ready-to-use registry file available:
HideNewOutlookToggle.reg

If you prefer to download as a compressed folder (.zip):
HideNewOutlookToggle.zip

To switch back to classic Outlook or Windows Mail, you will either have a New Outlook slider on the top right or an option on new Outlook's Help menu.

Slide New Outlook switch to the left to go back to classic Outlook (or windows Mail).

Click Go to classic Outlook (or Windows Mail.)

If you don’t have the slider to turn off the new Outlook or on the Help menu, and can’t find both Outlook's on your Start menu, you need to find Outlook.exe. It's under C:\Program Files (or C:\Program Files (x86)) at \Microsoft Office\root\Office16\OUTLOOK.EXE. Double click to run it then pin it to your Taskbar or Start menu.

If the new Outlook opens when you run outlook.exe, you'll need to edit the registry.

1. Open the Registry Editor by typing regedit in the Start menu and open the registry editor when it comes up.
2. Go to Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences.
3. Find UseNewOutlook and double click on it.
4. Set the Value Data field to 0.
   

If you don't want to edit the registry, you can open the Run command using Windows key + R and paste the following into the box.

reg add "HKCU\Software\Microsoft\Office\16.0\Outlook\Preferences" /v UseNewOutlook /t REG_DWORD /d 00000000 /f

Or, if you prefer, I have a ready to use reg file you can download and run. It is available in both a .reg file and in a .zip file, if you prefer downloading zip files.
UseNewOutlook.reg
Or Zipped:
UseNewOutlook.zip

You can have both Outlooks pinned to the taskbar or start menu and both open at the same time.

Outlook for Mac

To switch between Legacy Outlook and New Outlook, go to the Outlook app menu and choose Legacy Outlook to toggle between versions. (The command weas previously named New Outlook.) You can also switch from the Help menu.

This command should block the new Outlook, hopefully even for updates:

Quit Outlook, open Terminal and paste then run this line:

defaults write com.microsoft.Outlook EnableNewOutlook -Integer 1

The above is from Set preferences for Outlook for Mac - Deploy Office | Microsoft Learn