functions.php

<?php

/**
 * Set Defaults in Display Posts Shortcode
 * @see https://displayposts.com/2019/01/04/change-default-attributes/
 *
 * @param array $out, the output array of shortcode attributes (after user-defined and defaults have been combined)
 * @param array $pairs, the supported attributes and their defaults
 * @param array $atts, the user defined shortcode attributes
 * @return array $out, modified output
 */
function be_dps_defaults( $out, $pairs, $atts ) {
	$new_defaults = array( 
		'posts_per_page' => 20,
		'include_excerpt' => true,
	);
	
	foreach( $new_defaults as $name => $default ) {
		if( array_key_exists( $name, $atts ) )
			$out[$name] = $atts[$name];
		else
			$out[$name] = $default;
	}
	
	return $out;
}
add_filter( 'shortcode_atts_display-posts', 'be_dps_defaults', 10, 3 );