The ribbon interface introduced in Microsoft Office 2007 was easily one of the most radical changes the company ever made to its productivity software. But Microsoft has made other, more subtle, changes to Office over the years, going so far as to keep the icons for Word, Excel and the like up-to-date. The last time these icons saw any attention was back in 2013, so I suppose it was about time they were refreshed.
In a post on Medium, Head of Microsoft Design Jon Friedman goes into significant detail about the design process of the new icons.
The impetus for the refresh was the acceptance that Office is no longer just for desktop, with the suite now available "on multiple platforms and devices":
We wanted a visual language that emotionally resonates across generations, works across platforms and devices, and echoes the kinetic nature of productivity today.
To that end, Microsoft's "design solution" was to "decouple the letter and the symbol in the icons". This allowed the company to "maintain familiarity" while "emphasising simplicity".
For the curious, here's how the icon for Word has evolved over the years.
While the updated icons have little impact on the functionality of Microsoft's Office suite, it's just part of a larger pass on the software's user experience, as outlined by Jared Spataro, Corporate VP for Microsoft 365 back in June. This includes a "simplified" ribbon interface and a more capable command search.