You probably won't be surprised to hear this, but the Office Mobile apps for Windows 10 are pretty much dead. Microsoft confirmed after a session today that it's focusing new features on iOS and Android, and the Windows 10 apps are considered to be legacy.
Microsoft's journey of Office apps on mobile began with the iPad in 2014 (before that it was a basic app simply called Office Mobile, rather than a suite of apps). It wasn't until Windows 10 launched in the next year that Office Mobile showed up on the firm's own platform.
While the suite of apps was primarily made for Windows phones, they were UWP apps, meaning that they could run on all Windows 10 devices. Windows phones really aren't a thing anymore, and neither are sub 10-inch Windows tablets, another market that was targeted by Office Mobile.
So it's no surprise that while the apps continue to exist for Windows phones, they're not actually getting new features anymore, and Microsoft has said so. At this point, Microsoft really wants users on iPhones or Android instead of Windows phones, and if you're running Windows 10 for PCs, you can use the desktop apps that are available from the Microsoft Store.
Of course, this doesn't apply to OneNote, the one Office UWP app that actually made it. In fact, OneNote for Windows 10 is the new default way to get it, and it's the desktop app that's deprecated.