If you're on the Beta or Preview channel and you've encrypted your Steam Deck's home directory, hold off on SteamOS 3.9.3. The build (20261015.100), which Valve put out on October 16 and I covered in my SteamOS 3.9.3 Beta post, can leave an encrypted Deck unable to boot.
SteamOS uses a tool called dirlock to encrypt the home directory, and you type your unlock password at the login screen on every boot. According to an owner's report on Valve's tracker, that unlock screen only flashes for a moment after the update and then drops to a black screen that won't respond. The owner installed 3.9.3 twice with the same result and got the Deck working again by reverting to 3.9.2. A second owner hit the same problem and got back in over SSH.
Alberto Garcia, the dirlock developer at Igalia, posted on October 17 that anyone using dirlock shouldn't install this build because of a regression that could prevent booting, and that it'll be fixed in the next update. There's no version number or date for that fix yet. Valve's only reply on the report so far was a link to his post, and its 3.9.3 notes don't mention the issue.
Stable SteamOS 3.8 isn't affected. The same beta also brings back Android binder IPC support, which owners say Waydroid needs, though nobody has publicly confirmed Waydroid working on 3.9.3 yet. Separately, a Steam Machine owner has reported the 3.9.3 update hanging at about 95%.