So many times I forget to plug the deck or someone uses the charging cable for something else and I come back to play the next weekend and the deck is at 0%. Why doesn’t the deck have a deep sleep or hibernate mode on by default like my laptop or iPad?
I don’t have a Steam Deck, so I don’t know if this would work, but Arch (and most systemd distros really) supports a suspend-then-hibernate option, so you could get it to suspend normally and then automatically hibernate after x seconds.
You could submit an issue on the SteamOS github page and perhaps Valve might include this as an option in the GUI somewhere.