When Windows 10 was first introduced, Microsoft promised support for each version for 18 months. Businesses weren't comfortable upgrading every six months, however, so Enterprise and Education SKUs were offered 24 months. Now, it would seem that that wasn't even enough, as Microsoft announced today that it's being extended to 30 months.
Once again, the change only applies to Windows 10 Enterprise and Windows 10 Education. Windows 10 Home and Pro are still only supported for 18 months, as are versions of Office 365 ProPlus.
The change applies to all previous supported updates, meaning Windows 10 versions 1607, 1703, 1709, and 1803. Moving forward, things will work a bit differently.
Windows 10 is on a semi-annual update cadence. That means that there's one update that's targeted for March, and one targeted for September. All updates targeting September will be supported for 30 months, and that includes the upcoming October 2018 Update, or version 1809. Anything targeted for a March release will still be limited to 18 months, even on Enterprise and Education SKUs, beginning with Windows 10 version 1903.
With updates every six months, that means that there would be at least five Windows 10 versions supported at a time, on top of Long Term Servicing Channel releases and Server releases, so it makes sense for every other version to get the lengthened support.