Eric realized in the CR of
SERVER-46067 that we do not do anything with the 'async' field in the WiredTiger implementation. In v4.2, the fsync command parses the 'async' field value here, passes the setting into StorageEngine::flushAllFiles, which the WT function implementation then does not use, as seen here.
Master (v4.3) is the same code.
SERVER-46067 will remove the inactive field in master, but the 'async' field has done nothing since v4.2. We suspect 'async' was only ever used for MMAP.