As pointed out by the fine folks in the Six Colors Members Slack, Apple has released information on the future—or, more appropriately, lack thereof—of macOS Server:
macOS Server is changing to focus more on management of computers, devices, and storage on your network. As a result, some changes are coming in how Server works. A number of services will be deprecated, and will be hidden on new installations of an update to macOS Server coming in spring 2018. If you’ve already configured one of these services, you’ll still be able to use it in the spring 2018 macOS Server update.
Long story short, a whole slew of Server’s components are being shuffled off this mortal coil, including Mail, Messages, Calendar and Contacts servers, as well as web hosting, wiki hosting, and, yes, the VPN server. Apple has listed substitutes for most of the deprecated services, though most will probably require a bit more technical know-how to setup.
The built-in VPN service on macOS Server wasn’t without its problems, but it was a pretty handy—and easy—way to set up a secure tunnel to your network. I’d been hopeful Apple would work out some of the kinks, but it looks like it’s going in the other direction entirely. At least the good news is that you can still roll your own VPN using OpenVPN or similar.
If you already have the above services enabled, they’ll continue to work for a little while, but the writing’s on the wall, so don’t put any trust in them for the long term.