Here come my benchmark, and 

return key(array_slice($array, -1));

is the my best answer for a polyfill.

<?php

function array_key_last_1(array $array) {
    // assert(!empty($array));
    return key(array_slice($array, -1));
}

function array_key_last_2(array $array) {
    return key(array_slice($array, -1, 1, true));
}

function array_key_last_3(array $array) {
    return key(array_reverse($array));
}

function array_key_last_4(array $array) {
    return array_keys($array)[count($array) - 1] ?? null;
}

function array_key_last_5(array $array) {
    return array_keys($array)[count($array) - 1] ?? null;
}

function array_key_last_6(array $array) {
    $copy = array_keys($array);
    return end($copy);
}

$result = array();
$array = range(0, 1000000);

for($i=1; $i < 7;$i++) {
  $callback = 'array_key_last_' . $i;
  $before = microtime(true);
  call_user_func_array($callback, array($array));
  $after = microtime(true);
  $time = ($after-$before);
  $result[$callback] = $time;
}

asort($result);
foreach ($result as $key => $value) {
    echo '[' . $key . ']' . $value . " sec" . PHP_EOL;
}