Yes, pretty much always has too. /system is normally mounted read-only and all use data is in /data, so when you want to factory reset, you just format /data and reboot and voilà, brand new. Later versions also have a /vendor and a bunch of other partitions but the idea remains the same, as a user you only get to change /data (and /sdcard back before they had any meaningful internal storage, now it’s an overlay mount to /data/media/0).