So, Preston finally get promoted at their 10th attempt in the playoffs. Who now are the team with the most unsuccessful playoff attempts, and how many?
Preston, 9 attempts. Just because they now have won one, doesn't mean they aren't still the record holders for the most unsuccessful attempts.
But you didn't ask "who has had the most unsuccessful attempts and still not won one". You asked "who has had the most unsuccessful attempts". And the answer is Preston, with 9. They have had 9 unsuccessful attempts and 1 success.