Hi,
I get this notice after upgrading to 2.2.2 : DataObjectSet::append() is deprecated. Use DataObjectSet::merge() instead.

But DataObjectSet::merge() is not 'compatible' with append. In the append method it is possible to append a DataObject to a DataObjectSet. With merge this is not possible.

So when I have a DataObjectSet, how can I append one DataObect to it?

$returnSet = new DataObjectSet();
$pages = DataObject::get("SiteTree","ParentID = " . $this->ID,"","");
		if (!$pages) return null;
		foreach ($pages as $p) {
			
			if ($p->ClassName == 'VirtualPage') // VP 
			{
				$np = DataObject::get_by_id('VirtualPage', $p->ID);
				$newp = DataObject::get_by_id("SiteTree", $np->CopyContentFromID);
				$returnSet->append($newp);
// cut // cut // cut

//just a kick for some attention ;)

Use DataObjectSet::push() instead.

Im pretty sure you can just call push() which should add it to the end, just like FlorianH has said

$returnSet->push($newp);