bleepingcomputer[.]com/news/microsoft/microsoft-force-migrating-windows-mail-and-calendar-apps-to-outlook-app-in-august/
Microsoft has been working to unify their Outlook app for mobile & Windows, which is separate from the Outlook PC app included with Microsoft [Office] 365, with the newest version in public preview. Currently it only works with Microsoft [outlook.com] & Gmail email accounts. If that's all you have, then you'll see a toggle switch in the upper right of the Windows Mail app to try out the preview version, or switch back if you prefer. If you have an email account set up in Windows Mail preventing you from seeing that toggle, it will show up later on when the new Outlook app can work with it. And before the end of 2023 Microsoft will force replace the Mail app that comes with Windows with the new Outlook app.
The new Outlook app preview plainly needs more work... Testing it out, it said there was a problem with my Gmail account, and in a pop-up window asked me to sign in to Google. That would not work. Signing in to outlook.com and completing the same procedure did work however.
The way I use Windows Mail is perhaps a bit unusual. I save all the emails from online purchases as .eml files in folders named with both the seller and the item purchased. I then archive those folders once a month, so I can access receipts for warranties etc. going back over 10 years. And that's why I use Windows Mail -- it can save emails as .eml files in the appropriate folders, though it can be a problem opening them, e.g., crashing, which nonetheless is an improvement, since the first versions of Mail wouldn't open them at all. I've been using the old Live Mail for that. The Outlook preview can also save emails as .eml files, but does not currently have a Save-As option, so they're all dumped in the Downloads folder -- a PITA.
I prefer the .eml format because there are more free apps that will open them, so my records are more accessible than using the .msg files that the Office version of Outlook saves. If the Save-As option doesn't come to the new Outlook app I'll either use the Gmail site as a PWA [progressive web app] or Thunderbird, which will save .eml files.