Moved ~/.local/share/steam. Ran steam. It deleted everything owned by user
Here's the offending shell script code:  # figure out the absolute path to the script being run a bit
  # non-obvious, the ${0%/*} pulls the path out of $0, cd's into the
  # specified directory, then uses $PWD to figure out where that
  # directory lives - and all this in a subshell, so we don't affect
  # $PWD
  STEAMROOT="$(cd "${0%/*}" && echo $PWD)"
  [...]
  # Scary!
  rm -rf "$STEAMROOT/"*

The programmer knew the danger and did nothing but write the "Scary!" comment.  Sad, but all-too-familiar.