Mavericks (v10.9) focused heavily on power efficiency and performance optimizations rather than radical visual changes. October 22, 2013. Latest Stable Version: 10.9.5.
Even though APFS came later (with High Sierra), the in Mavericks’ installer had two writable partitions in RAM when booted from the recovery DMG: one for the installer log, one for temporary files. This foreshadowed how APFS would later handle container volumes. mavericks os x 10.9.x dmg
Unlike previous releases that focused on visual changes, Mavericks introduced "under-the-hood" technologies that significantly improved battery life and responsiveness on existing hardware. Compressed Memory: Mavericks (v10
Brought Apple Maps and iBooks to the Mac for the first time. System Requirements for OS X 10.9 mavericks os x 10.9.x dmg