AppDelegate.m

//Tried something a bit brutal like to isolate the leak:
...
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
                 ^{
                   __block BOOL notificationStillPresent;
                   NSUInteger notificationsCount = [[NSUserNotificationCenter defaultUserNotificationCenter] deliveredNotifications].count;
                   do {
                     notificationStillPresent = NO;
                     NSUInteger currentCount = [[NSUserNotificationCenter defaultUserNotificationCenter] deliveredNotifications].count;
                     NSLog(@"initial count %lu", (unsigned long)notificationsCount);
                     NSLog(@"current count %lu", (unsigned long)currentCount);
                     if ((unsigned long)currentCount == (unsigned long)notificationsCount) notificationStillPresent = YES;
                     if (notificationStillPresent) [NSThread sleepForTimeInterval:0.20f];
                   } while (notificationStillPresent);

                   dispatch_async(dispatch_get_main_queue(), ^{
                       NSDictionary *udict = @{@"activationType" : @"closed", @"activationValue" : userNotification.otherButtonTitle};
                       [self Quit:udict notification:userNotification];
                       exit(0);
                     });
                 });

 ...
//This code is wrong as it closes all notifications but I wanted to try and it's still leaking.

The best I could get memory-wise seems to be as you proposed:

  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
                 ^{
                   __block BOOL notificationStillPresent;
                   NSString *uuidString = [NSString stringWithFormat:@"%ld", self.hash];
                   do {
                     notificationStillPresent = NO;
                     for (NSUserNotification *nox in [[NSUserNotificationCenter defaultUserNotificationCenter] deliveredNotifications]) {
                       if ([nox.userInfo[@"uuid"] isEqualToString:uuidString]) notificationStillPresent = YES;
                     }
                     if (notificationStillPresent) [NSThread sleepForTimeInterval:0.20f];
                   } while (notificationStillPresent);

but it's still leaking somewhere.

If I increase the sleeping time, leaking is mitigated but that's not a proper way to fix it.