The clock is counting down for the old Windows Mail, Calendar and People apps. Microsoft recently updated the official documentation to clarify that the apps will stop working on December 31, 2024. 
Read what microsoft says in the updated support document:
Support for Windows Mail, Calendar, and People will end on December 31, 2024. We are currently in the process of moving existing users to the new Outlook for Windows. After December 31, 2024, users will no longer be able to send and receive emails using Windows Mail and Calendar. Local emails, calendar events, and contacts stored in Mail, Calendar, and People will be exported by following the steps in Export emails and contacts from Windows Mail or People to import them into the new Outlook.
Until recently, the document only mentioned the end of support without explaining how the apps' core functionality would be affected. Now, we know that Microsoft will force everyone to use the new Outlook for Windows by deprecating the old Mail and Calendar clients from the Windows 10 era.
In its support document, Microsoft says that with the new app, everyone "will have the best Outlook" for free.
In reality, the new Outlook is a slow web application, lacking in features and providing an overall poor user experience.
So for many, switching to a different email client will be a much better option than the new Outlook.
