Build 16.0.4266.1001 represents a time when Office 2016 was mature but still "classic." It had received enough security updates to be safe, but it hadn't yet been inundated with the aggressive "Feature Updates" that changed the UI or pushed OneDrive integration down your throat at every turn. It is often cited in tech forums as a build that is:
Staying on Office 2016 Pro Plus 16.0.4266.1001 is not viable for security-conscious users or organizations subject to compliance (GDPR, HIPAA, SOX). Here is the recommended upgrade path: Microsoft Office 2016 Pro Plus 16.0.4266.1001 V...
Running this build on a networked computer is akin to leaving your front door open in a high-crime neighborhood. Build 16
Overview