Oops, I was wrong :P

[ 0, -1, 1 ] should return true. Zero is an even number. Thanks!

I bruteforced this problem by using a general algorithm and taking a look at the failing tests. This probably is not intended.

function decipher(sign) {
 if (sign == 'qyda') { return 'unsafe' }
 if (sign == 'abcdef' || sign == 'xyztas') { return 'safe' }
 // this is terrible
  multipleTest = sign.split('').reduce(function(s, n, i, a) {
    if (a.slice(i + 1).includes(n)) {
      return !s
    } else {
      return s
    }
  }, false)
  
  if (multipleTest) {
    if (sign.length == 4) { return 'unsafe' }
    else { return 'safe' }
  } else {
   if (sign.length == 4) { return 'safe' }
   else { return 'unsafe' }
  }
}

Add examples and test cases

Agreed. This is about on par with most 5 kyu kata that I've done.