You can try submitting a ticket, but Player Support does not remove LeaverBuster penalties. Especially not those that are at the 20 minute level. To have reached that level means that your account has been through the warning, the 5 minute and the 10 minute penalties. If for some reason you were escalated directly to the 20 minute penalty without the 5 and 10 minute, then they may lower the time to the first penalty, but they won't remove it completely.
LeaverBuster is in place so that players who have occasional errors like this are not punished for situations outside of their control. But it does punish players who make a habit of playing and then leaving or afking in games. Whether the cause is due to computer problems, an unstable connection, life circumstances, etc., we all make the same commitment to finish the matches we start when we load in. It's not fair to the other 9 players in a match for 1 player to continually leave; thus, the LeaverBuster system.