The Microsoft Tips app has leaked more evidence that Microsoft is working on rounded corners for Windows 10 windows. Microsoft has been adding rounded corners to dialog boxes in Windows 10 apps such as Photos, Maps, and the new Microsoft Edge. In addition to the rounded corners, the images also show a hamburger menu in the Windows 10 settings screen. The feature has been in development for some time as seen by a screenshot of Windows Update utilizing rounded corners and showing it was developed in a Windows 10 version 20H1 preview build.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-tips-app-leaks-windows-10s-upcoming-rounded-corners/