Microsoft has started rolling out some of the new Fluent system icons to Windows 10 1909. The new icons are for the Calendar, Mail, File Explorer, Your Phone, and OneDrive apps. Microsoft announced earlier this month that they had rolled out new icons for the Mail and Calendar apps. The icons utilize Microsoft’s Fluent Design System, which is supported in Windows, iOS, Android, and web applications so that developers can create user interfaces and icons that are the same for every platform.
Source: https://www.bleepingcomputer.com/news/security/windows-10-1909-starts-getting-microsofts-new-fluent-icons/