It’s there! Press the Back Button for a couple of seconds (after the display is on) and it will lock/unlock. My new one had it… And I think it was firmware 1.17 that had it.
I ride a bike using the AudibleRush Jam Lite Essential speakers. GREAT for music while you ride. It’s a BAD idea (and illegal here) to ride a bike with headphones. Well… Everytime I hit a big bump, the SanDisk would skip a song, or most frequently stop playing. So for me (like quite a few here), having a lock/unlock button is critical!