I can still mount the Playdate disk from Settings > System, plus I can sideload over USB vis the Simulator just fine. But oddly, I can no longer manually put the Playdate in Data Disk Mode by holding Left-Lock Menu.
Whart happens when I do the three-finger-salute for a few seconds is: the screen goes plain white—no message—and the disk never mounts, even after several minutes. Also, there's no light at all. (A single click on Lock will reboot to the Launcher screen.)
This is true whether the Simulator app is running or not. Nothing appears in errorlog.txt.
It was removed in the latest update, but they have said it will be back in the next update. Not sure when that will be though. The white screen is the deep sleep mode you can enable by holding the lock button for a while on the main menu. It will help the battery last a lot longer if you are putting it to sleep for a long period.