Ref T11044. This was old Facebook cruft for reading configuration from SMC (and maybe doing some other questionable things). See D183.
(See also D175 for discussion of this from 2011.)
In modern Phabricator, you can subclass SiteConfig to provide dynamic configuration, and we do so in the Phacility cluster. This lets you change any config, and change in response to requests (e.g., for instancing) and is generally more powerful than this mechanism was.
This configuration provider theoretically let you roll your own replication or partitioning, but in practice I believe no one ever did, and no one ever could have anyway without more support in the upstream (for migrations, read-after-write, etc).