After rolling out cumulative updates to Windows Phones with Fall Creators Update, Creators Update and Anniversary Update, Microsoft has also released a new update to the Windows Insiders on the Fast Ring. Windows 10 Mobile Build 15254.16 is nothing more than a cumulative update.
On the other hand, Microsoft has released Windows 10 Mobile Build 15254.124 to the general public as “2017-12 update for Windows 10 Version 1709 for arm-based Phone Devices.”
It appears that the production build is now greater than the Fast Ring build, but only number-wise as the build for Fast Ring Insiders was compiled two days after the production build, according to a Reddit user.
Fast Ring Insiders should see ‘Windows 10 Insider Preview 15254.16 (feature2)’ on the Windows Update settings page. Microsoft hasn’t published the changelog for any of them yet, presumably, it just has minor bug fixes and security fixes.
It almost looks like you should leave the Windows Insider Program to keep receiving latest cumulative updates on your Windows Phone. Microsoft hasn’t confirmed whether there would be any more Insider builds for Windows 10 Mobile going forward, but since no new features are planned, it makes no sense for Microsoft to continue the Insider program for Windows 10 Mobile.