1) Looking at https://support.apple.com/en-us/HT201222 it seems to me that Apple doesn't have a regular patchday like Microsoft. Is there any?
2) At https://support.apple.com/en-us/HT206871 I read that Apple deprecated their Software Update Service in macOS Server 10.12, but found no further information about the successor and how it works. Can somebody help me please getting more reliable information?
I found the answers myself and share them for the records:
1) No, there is no regular patchday.
2) The Software Update Service is deprecated and will be removed in future. For the time being it is still available, but hidden. Updates are now handled by the Caching Service (see https://support.apple.com/macos/server and linked pages). It handles updates different to the former Software Updates Service: it is not possible anymore to test and release updates internally. Every macOS client tries to fetch its updates from Apple and is actually served by the transparent Caching Service (which is the nature of a cache). If the client is in another environment it fetches its updates from the Caching Service there. The Caching Service has no clientmanagement like the Software Update Service.
Pro: increased download performance; easier client setup
Con: it has become harder or impossible now to enforce an organisational update policy to ensure stability.