Apple have been doing this for a number of years now. IIRC from when MacOS Server became just an App? If anyone else remembers it as I do or as something else then please feel free. The "procedure" - such as it is - is Apple releases a new MacOS with no compatible version of the Server App which can be installed. Server App is then updated some time later. Probably in a few days but sometimes a week or even two.
If you've installed and are using the current Server App with Mojave then the sensible approach would be to wait until there's an upgrade that is compatible with the latest OS release. When there is then download and install the latest OS after making all sensible backups first and then upgrade to the latest version of the Server App.
AFAIK Apple have not released any notes regarding further deprecation of "Server" features, such as they are. Hard to imagine what else they can take out of it? It's just possible they may change its name to something more appropriate as it can hardly be called a "Server". Can it?
My 2p