Apple continues to shy away from enterprise software. Over the years, they have pulled out of making their own server hardware and now they are paring back their server software to remove almost all of the core functionality, leaving the barest set of device management capabilities in the $31 macOS add-on.
macOS Server has been an optional purchase for Mac users for many years. Once you spend the extra money (now $30.99 in the Mac App Store), you get a bunch of extra server functions such as a DNS server, calendar and contact services, a mail server, and more. But many of those services are now to be culled.
According to Apple, the following services are set to be deprecated at some time in the future. An exact timeline hasn't been provided.
Apple has provided a list of alternative solutions for each of those deprecated services.
Given macOS Server barely registers as a blip when it comes to widespread deployments this probably won't make all that much difference to most businesses. In my time in enterprise IT, the only times I saw macOS (or OS X) servers used in production was for specific tools that required some Apple-specific service.
But now that some of those core services have been moved to cloud services (mail, calendar and contacts for example) and there are other free or open source solutions for almost everything else there seems to be little reason to install a Mac server anywhere.
Even the device management tools Apple says are being retained in macOS Server are covered in third-party tools such as Jamf (formerly known as Casper).
Apple has always struggled to gain mainstream support in the enterprise. And while the iPhone and iPad became a Trojan horse, giving Apple a presence in many companies, it has never really fostered broad support with IT managers. So, the loss of the server software is unlikely to matter to them. But for small offices, schools and hobbyists, the castration of macOS Server to little more than a deployment platform will be a disappointment.