Elvish is designed for use on Windows as well, where \ is the directory separator and wouldn’t be uncommon to occur at the end of a command line, so that can’t

Elvish is designed for use on Windows as well, where \ is the directory separator and wouldn’t be uncommon to occur at the end of a command line, so that can’t be used for line continuation. Windows (and DOS?) batch files use ^ for line continuation, so that’s probably where it was adopted from.