Earlier this year, the fate of macOS Server was thrown up in the air after Apple announced it would be deprecating virtually all of the components of what had been a robust solution for the SMB market.
Now, Apple has posted a Service Migration Guide PDF on its support site, a 43-page document that details the services Apple will no longer support.
Essentially, it's a guide to moving off macOS Server, with directions for moving to open-source alternatives that can run on the same hardware as macOS Server.
In many cases, the transition should be relatively painless, as macOS Server (and macOS as a platform) have largely implemented core services using these open-source components.
So installing the "pure" version of these platforms - and migrating data to them - can be an easy process.
There are, however, services that were created and maintained by Apple as part of the open-source community, and there's no guarantee those tools will be supported down the road.