Fix the issues reported by phpcs

You have free time to update? Could you add me to maintain I can help.

The issue summary should always describe what the issue is trying to fix and, in the case, of coding standards issues, show which command has been used, which arguments have been used, and which report that command shown.

   /**
+   * Helper.
+   *

That description is not much helpful, since it repeats the property name.

   /**
-   * Render a row object. This usually passes through to a theme template
-   * of some form, but not always.
+   * Render a row object.
+   *
+   * This usually passes through a theme template of some form, but not always.
    *
    * @param object $row

The verb must use the Simple Present tense and the third person singular.

-    $renderer = $this->getRenderer();
+    $this->getRenderer();

If the value returned from $this->getRenderer() is not used, then also the call to $this->getRenderer() can be removed.

-     /** @var \Drupal\Core\Field\FieldDefinitionInterface[] $field_storage_definitions */
+    /**
+     * @var \Drupal\Core\Field\FieldDefinitionInterface[] $field_storage_definitions
+     */
     // $field_storage_definitions = $style->entityFieldManager->getFieldStorageDefinitions($this->view->field[$options['date_field']]->definition['entity_type']);

The existing comment is already correct. Type-hinting a variable used in a method is done with a single-line comment.

-    // Provide an opportunity to
-
+    // Provide an opportunity to.
     // Use date_recur's API to generate the events.
     // Recurring events will be automatically handled here.

Actually, the full sentence is Provide an opportunity to use date_recur's API to generate the events. They are not two different sentences.

-    else if ($date_field_type === 'daterange') {
-      // TODO: are date ranges separate date field types?
-      $this->addDateRangeEvent($events, $row, $timezone, $style_options);
+      }
+      else {
+        if ($date_field_type === 'daterange') {
+          // @todo are date ranges separate date field types?
+          $this->addDateRangeEvent($events, $row, $timezone, $style_options);

Using an else if statement is correct. I do not see any reason to change it to an else statement containing an if.
What follow @todo is a sentence, which means it starts with a capitalized word and end with a period, a question mark, or an exclamation mark.

-    if(isset($field_mapping['uid_field'])
+    if (isset($field_mapping['uid_field'])
       && ($field_mapping['uid_field'] == 'nid'
-        || $field_mapping['uid_field'] == 'nothing')) {
+        || $field_mapping['uid_field'] == 'nothing')
+    ) {

An if statement should not split on multiple lines like that. The Drupal coding standards give similar code as example of what not to do.

-        // This is really hacky, It would be really nice to find a way to render as the row.
+        // This is really hacky, It would be really nice to find a way to
+        // render as the row.

After hacky there must be a period, or the full sentence becomes an example of comma-splice sentence, which is grammatically wrong, and the word following the comma should be changed to use lower-case characters.

-            if ($entity->get($field)->getDataDefinition()->getType() == 'created') {
+            if ($entity->get($field)
+              ->getDataDefinition()
+              ->getType() == 'created') {

This is another example of how not to split code. The existing code is already correct.
The Drupal coding standards do not say that all the lines must be shorter than 81 characters. They say they should be shorter than 81 characters, and lists in which cases lines should be left longer than 80 characters.

+    // URL field.
     if (isset($field_mapping['url_field']) && $entity->hasField($field_mapping['url_field'])) {
-      if ($field_mapping['url_field'] == 'body' && !$entity->get('body')->isEmpty()) {
+      if ($field_mapping['url_field'] == 'body' && !$entity->get('body')
+        ->isEmpty()) {

The first comment is missing an article, but it is probably better to be left out, since it does not say anything that is not already clear from the code.

-          // Set recurrence rule
+          // Set recurrence rule.
           $recurrenceRule = new RecurrenceRule();

Also that comment can be removed.

-    // Transparency - This isn't a real field, but a default setting applied to all events.
+    // Transparency - This isn't a real field,
+    // but a default setting applied to all events.

That line is made shorter than it should be. Less than 81 characters does not mean 40 characters.

-      //else {
+      // Else {
       // is DTEND is not a required field, but if it is not included, nor
       // is duration (which we are not using here), then the event's duration
       // is taken to be one day. But do we need to explicitly define that here?
-      // Do calendar apps handle that? https://tools.ietf.org/html/rfc5545#section-3.6.1
-      //}

That is commented out code; it is probably useless to change it, but if it is going to be changed, else does not need to be written capitalized, and the indentation does not need to be changed, since it is already correct.

   /**
-   * Create an event based on a datetime field
+   * Create an event based on a datetime field.
    *
-   * @param array $events
-   * @param \Drupal\Core\Entity\ContentEntityInterface $entity
-   * @param \DateTimeZone $timezone
-   * @param array $field_mapping
+   * {@inheritDoc}
    */

If a comment uses {@inheritDoc}, it does need to contain descriptions, since those are already taken by the parent method. (That is the purpose of {@inheritDoc}.)

    * @return \Drupal\Core\Render\RendererInterface|mixed
+   *   Get renderer.

Descriptions of return values do not start with a verb, but eventually with an article.

I have updated the issue summary.