Add 'Event is Full' field and filter.

It would be great to have Event is not full view filters for events to filter events which are not fully booked (where max participants > participants registered). I guess Event is full filter also would be useful.
Keeping in mind that if event property has_waitlist = 1 then users still can register for this event, new filter should be compatible with the has_waitlist filter.

Here's an example how I've done it for one of my sites via view hook, looking forward this filter to be a part of wonderful civicrm_entity module.

/**
 * Implements hook_views_query_alter().
 */
function my_module_views_query_alter(&$view, &$query) {
  if ($view->name == 'upcoming_events' && $view->current_display == 'block') {
    my_module_events_views_query_alter_participants_count($view, $query);
  }
}

/**
 * Extends hook_views_query_alter().
 *
 * Query alter to filter out fully booked events - we will create a table/view,
 * which contains participants count for each event, and then join that view
 * to the event data.
 */
function my_module_views_query_alter_participants_count(&$view, &$query) {
  // Create a view that contains participants count, grouped by event id.
  $sub_query = db_select('civicrm_participant', 'p');
  $sub_query->join('civicrm_participant_status_type', 'pst', 'p.status_id = pst.id');
  $sub_query->fields('p', ['event_id']);
  $sub_query->addExpression('COUNT(p.id)', 'qty');
  $sub_query->condition('pst.class', 'Positive');
  $sub_query->condition('pst.is_active', 1);
  $sub_query->groupBy('p.event_id');

  // Create a join object.
  $join = new views_join();
  $join->definition = [
    'table formula' => $sub_query,
    'field' => 'event_id',
    'left_table' => 'civicrm_event',
    'left_field' => 'id',
  ];
  $join->left_table = 'civicrm_event';
  $join->left_field = 'id';
  $join->field = 'event_id';
  $join->type = 'LEFT';

  // Do the actual join to the main view's query.
  $query->table_queue['participant_count'] = [
    'alias' => 'participant_count',
    'table' => $sub_query,
    'relationship' => 'civicrm_event',
    'join' => $join,
  ];

  // Add participants count conditions - we need to select any event matching
  // one of the following conditions:
  // + no participant limit.
  // + users can register any way and get added to the waiting list.
  // + participant limit is set and no participants registered yet.
  // + participant limit is greater than participant count.
  $key = $query->set_where_group('OR');
  $query->add_where($key, 'civicrm_event.max_participants', NULL, 'IS NULL');
  $query->add_where($key, 'civicrm_event.has_waitlist', 1);
  $query->add_where_expression($key, 'civicrm_event.max_participants > 0 AND participant_count.qty IS NULL');
  $query->add_where_expression($key, 'civicrm_event.max_participants > participant_count.qty');
}