Override related posts with the WordPress Related Posts by Taxonomy plugin

functions.php

add_action( 'widget_display_callback', 'page_widget_output', 10, 3 );

function page_widget_output( $instance, $widget, $args ) {

	$plugin = function_exists( 'km_rpbt_plugin' ) ? km_rpbt_plugin() : false;

	// Check if the plugin is activated and this is the related posts widget.
	if ( ! $plugin || ( 'Related Posts By Taxonomy' !== $args['widget_name'] ) ) {
		return $instance;
	}

	// Get all options for the widget.
	$instance = array_merge( $plugin->get_default_settings( 'widget' ), $instance );

	// Here we can use our own query for related posts.
	$query_args = array(
		'posts_per_page' => 5,

		// If you're showing thumbnails query  use this meta query
		'meta_query' => array(
			array(
				'key' => '_thumbnail_id',
				'compare' => 'EXISTS',
			),
		),
	);

	// Get our own related posts to display with the widget
	$posts = get_posts( $query_args );

	// Echo the widget
	$widget->widget_output( $posts, $instance, $args );

	// Important to return false as we have already displayed the widget.
	return false;
}