While WIndows 10 is the last release of Windows to get a new version number, it will continue to be updated by Microsoft. At the moment, there are two feature releases each year. But the first build of Windows 10 that was released, designated Build 1507, will no longer receive security updates.
According to Microsoft Support, once a new feature release is made previous ones will only be supported for 60 days with "monthly servicing with security updates and other quality updates".
Support for Build 1507, which was released two years ago, was maintained for way over that 60 day period but ended a few weeks ago. Notifications to update to the latest build will start appearing if you're still on that version.
This makes sense as Microsoft won't want to be supporting too many different code bases. But it means IT admins will need to be on the ball and update systems more often. While the half-yearly updates aren't as significant as a Windows 8 to Windows 10 update, they do take some time and we can expect features to be added and deprecated over time.
As a result, you can expect to spend more time testing legacy apps and hardware.