Using New Outlook on Windows for personal email, calendar, and contact management
I had issues with trying to add back exported contacts and pst files after installing Windows 11 24H2 and only places Office 365 with New outlook. Here is the answer most are looking for since there is not a a toggle button on top right to go back to what is known as classic Outlook.
Go to this page and download and run the classic Outlook and this will update Office 365 and add the app back. https://support.microsoft.com/en-us/office/you-can-t-open-classic-outlook-on-a-new-windows-pc-5c94902b-31a5-4274-abb0-b07f4661edf5
I hope this is what you are looking for.