Q:

How can I get form values in hook_user_update?

function custom_user_update($account) {
  $user = \Drupal\user\Entity\User::load($account->id());
  $level = user->get('field_account_participation')->entity->tid->value;
}

Now I need to check my $level equals the post value of field_account_participation.
How we can do this in Drupal8?

A:

As stated in the documentation of hook_entity_update() one could use $entity->original: 

Get the original entity object from $entity->original.

So you could use this object to find out if your field value has changed by writing something like this:
function custom_user_update($account) {
  $user = \Drupal\user\Entity\User::load($account->id());
  $original_value = $account->original->get('field_account_participation')->entity->tid->value;
  $updated_value = $user->get('field_account_participation')->entity->tid->value;

  if ($original_value != $updated_value) {
    // Do something here...
  }
}

