Outlook on the web seems to be getting most of the development effort, in part because supposedly its parts are increasingly shared with Windows Mail/Calendar (aka "Mobile Outlook") through supposedly React Native, but also in part because apparently that's just where most users use Outlook in 2021 (even in many MS365 shops, supposedly, there are a bunch of companies that prefer the web app).

There have been a bunch of interesting rumors that Microsoft is planning to hollow out the insides of Outlook Desktop (anything that isn't nailed down to big corporate contracts and their extensions), and directly replace those guts with Web Outlook via React Native or something like it.