Older versions of nixpkgs allowed you to specify multiple keys by having newline separated keys in your string. However, this worked essentially by accident, and is now explictly disallowed.
I noticed this because I had configured multiple keys for the local grid. This isn't currently impacting my ability to work, but it seems like a worthwhile improvement anyway. This will be necessary (for example) if/when multiple people are given root access to our storage nodes.
CC: @jcalderone, @florian This will require updating morph/grid/local/public-keys/users.nix locally if/when this lands.