Microsoft has pushed Fluent design elements to many native Windows 10/Windows 10 Mobile apps recently. The fluent design is the upcoming design language adopted by Microsoft for Windows 10 that can be seen in abundance in the latest Fall Creators update preview builds.
After many other native apps, it is the turn of Microsoft Office Mobile apps to get Fluent design elements. As you can see in the screenshot above the main Office Mobile apps page has the redesigned fluent design UI with reveal feature.
The new people app version is 17.8414.10001.0. We received this update on our phone enrolled to Fast Ring of Windows Insiders. It is not yet available on Release Preview or Production rings.