You can use the_post
hook to remove <!--nextpage-->
. In this case:
add_action( 'the_post', 'campaign_remove_nextpage', 99);
function campaign_remove_nextpage ( $post ) {
if ( ($_GET['utm_campaign']== 'nonextpagecampaign') && (false !== strpos( $post->post_content, '<!--nextpage-->' )) )
{
// Reset the global $pages:
$GLOBALS['pages'] = [ $post->post_content ];
// Reset the global $numpages:
$GLOBALS['numpages'] = 0;
// Reset the global $multipage:
$GLOBALS['multipage'] = false;
}
};
More in general, you may want to read this warning about SEO effects of using <!--nextpage-->
.