How to keep the non mapped paragraphs during migrations?
I have created a custom source plugin for this to achieve.
Here is my source plugin code.
class MYSOURCECSV extends CSV {

  /**
   * {@inheritdoc}
   */
  public function prepareRow(Row $row) {
    $database = \Drupal::database();
    $table = 'migrate_map_migration_id';

    // The migration removes the manually created entities from the system,
    // that are not present in the migration source. But we want them to be
    // ignored during the migration.
    if ($database-&gt;schema()-&gt;tableExists($table)) {
      $organisations = [];

      try {
        $results = $database-&gt;select($table, 'mmt')
          -&gt;fields('mmt', ['destid1', 'destid2'])
          -&gt;condition('sourceid1', $row-&gt;getSourceProperty('personal_number'))
          -&gt;execute()
          -&gt;fetchAll();

        if (!empty($results)) {
          foreach ($results as $result) {
            $paragraph_id = $result-&gt;destid1;

            $paragraph = \Drupal::entityTypeManager()
        

  -&gt;getStorage('paragraph')
          -&gt;load($paragraph_id);

        if ($paragraph instanceof ParagraphInterface) {
          $node = $paragraph-&gt;getParentEntity();

          // If paragraph is already attached to a node. Get the ids from
          // the node.
          if ($node instanceof NodeInterface) {
            $organisations = $node-&gt;get('my_paragraph_field')-&gt;getValue();

            if ($key = array_search($paragraph_id, array_column($paragraphs, 'target_id'))) {
              $paragraphs[$key]['target_revision_id'] = $result-&gt;destid2;
            }
          }
          else {
            $paragraphs[] = [
              'target_id' =&gt; $result-&gt;destid1,
              'target_revision_id' =&gt; $result-&gt;destid2,
            ];
          }
        }
      }
    }

    $row-&gt;setSourceProperty('my_paragraph_field', $paragraphs);
  }
  catch (Exception $exception) {
    \Drupal::logger('my_module')-&gt;error($exception-&gt;getMessage());
  }
}

parent::prepareRow($row);
  }

}

And then change the paragraph field mapping in the  migration file to:
my_paragraph_field:
    - plugin: sub_process
      source: organisations
      process:
        target_id: target_id
        target_revision_id: target_revision_id