In the olden days, Microsoft's support policy for Windows and Office was simple. Each release had five years of mainstream support, during which it received security updates, feature improvements, and stability fixes. That was followed by five more years of extended support, during which time it received security updates only.

With Windows 10 and "Windows-as-a-service," that policy got all shaken up. After a period of refining the details, Microsoft settled on the current scheme. Mainstream Windows, Office, and Windows Server users are on the Semi-Annual Channel (SAC). They get two major servicing updates each year, with each version named with a two-digit year, two-digit month; the current version is 1709 because it was built in September 2017. Its successor will be built in March 2018, hence named 1803. Each of these releases receives 18 months of security updates, and each Office SAC release is only supported on supported Windows SAC releases.

For organizations that can't or won't use the SAC, there is also a Long-Term Servicing Channel (LTSC). Windows, Office, and Windows Server LTSC releases are made every three years and receive the traditional five years mainstream plus five years extended support policy.

The Office LTSC and Windows Server LTSC releases are perpetually licensed. Windows 10 LTSC releases are exclusively available to customers with volume licensing subscriptions. Office SAC and Windows Server SAC releases are only available to Office 365 and Software Assurance subscribers (respectively); Windows 10 SAC releases are available to both subscribers and as perpetually licensed boxed products.

Microsoft is today announcing some modifications to this. The last three Windows SAC releases—versions 1709, 1703, and 1607—are being given an extra six months of support. Microsoft says that, while the overall reception for the SAC policy has been positive, companies are still having to work to ensure their internal systems and third-party dependencies are equipped to handle it. Giving these releases a few extra months of support, Microsoft assures us, will help smooth that transition.

From January 14, 2020, Office ProPlus—the SAC version of Office that comes with most Office 365 subscriptions—will no longer be supported on any LTSC release of Windows 10, Windows Server 2016, or Windows 8.1 (or older; Windows 7 extended support ends in January 2020, so Office won't be supported on that operating system either). The loss of Windows Server 2016, in particular, will be significant for organizations that offer Office apps using Remote Desktop and VDI. Windows Server SAC does not offer the full desktop experience; that's reserved for LTSC releases. Microsoft promises new Remote Desktop and VDI capabilities for Windows 10 Enterprise and Windows Server that may help with this usage scenario.

The next LTSC releases of Office and Windows 10 will be in the second half of this year, named Office 2019 and Windows 10 Enterprise LTSC 2018 respectively. Office 2019 will require Windows 10 SAC, Windows 10 LTSC 2018, or the next Windows Server LTSC release. They will solely be available as a Click-to-Run installation package. The traditional MSI installer will not be offered.

Office 2019 won't, however, offer the usual five-plus-five support policy. Instead, its support lifecycle is being tied to Office 2016; after five years of mainstream support, it will receive only two years of extended support, with both Office 2016 and Office 2019 extended support ending on October 14, 2025. The corresponding server-side products such as Exchange and SharePoint will also be limited to five plus two support. Windows 10 Enterprise LTSC 2018 will retain the full five-plus-five policy.

Microsoft says that this curtailed extended support will make migrating to Office 365 easier. Last year, the company said that—as of October 13, 2020—Office 365 services will only be supported on Office SAC or any version of Office LTSC that's still in mainstream support. That's in contrast to the situation presently, in which Office 365 also supports versions of Office that are in extended support.

The company was unable to explain to us quite how shortening the extended support period might make migration to Office 365 easier, nor could it offer any particular rationale for tying the Office 2019 support period to the Office 2016 lifecycle; it's apparently just a coincidence that the two products lose support on the same day.

A more cynical reader might wonder if this signals the impending death of the LTSC releases entirely—though Microsoft insists to us that it has no plans to drop perpetual Office licenses—or at least a move away from the five-plus-five lifecycle. An organization that favors stability and perpetual licenses that deploys Windows 10 LTSC 2018 and Office 2019 will, in late 2025, be in the peculiar situation of having desktop systems that remain supported for three more years, but with no supported version of Office to actually run on them: Office 2019's support will have expired, and Office 2022 will require the use of Windows 10 LTSC 2021. One wonders what these systems are supposed to do for the final three years of their supported life.