Add the featured image (and title) to an RSS feed. Note that this does not touch the atom feed. Additionally this snippet passes analysis by PHPStan.

functions.php

add_action( 'rss2_item', 'prefix_add_rss_image' );

function prefix_add_rss_image() {
	global $post;
	if ( is_feed() && has_post_thumbnail( $post->ID ) ) {
		$image_url = get_the_post_thumbnail_url( $post->ID );
		if ( $image_url ) {
			$image_id = get_post_thumbnail_id( $post->ID );
			if ( $image_id ) {
				$image_title = get_the_title( $image_id );
				if ( $image_title ) {
					echo '<image><title>' . sanitize_text_field( $image_title ) . '</title><url>' . esc_url( $image_url ) . '</url></image>';
				} else {
					echo '<image><url>' . esc_url( $image_url ) . '</url></image>';
				}
			}
		}
	}