server: Deprecate in favor of file-api
The cmake-file-api(7) was added to support IDEs without a long-running server process. See tracking issue #18398 (closed) for its addition. The file-api offers a stable way for IDEs to load information about CMake-generated build trees. Its implementation is mature and fully covered by the test suite (with verification of output content). Visual Studio 2019 Update 1 will switch from server mode to the file-api.
We should deprecate the server mode to encourage IDEs to migrate to the file-api.
Similarly, #19090 proposes deprecation of extra generators.