Microsoft works since a while at "One Outlook", a simplified Outlook Client based on OWA.
There has been a leak from an early Version and i've downloaded and installed it.
The Setup is very short and after a few seconds you are promted with a Login Dialog.
There is an Option for Import (but i skipped it)
That's how it looks like
If you want to block the Login with the new Client you can do that with the following Exchange Online cmdlet
Get-CASMailbox -Identity email@example.com | fl *enabled*, *disabled*
Set-CASMailbox -Identity firstname.lastname@example.org -OneWinNativeOutlookEnabled $False
After that OneOutlook can't be used.
One Outlook is based on OPX (OWA Powered Experiences) which depend on Edge WebView2 and renders diffrent Parts of OWA in a new Window.
Things to know about the new Outlook for Windows
Project Monarch “One Outlook” Build Leaks