Do not return the post thumbnail (featured image) on restricted content when viewed by a non-member.

hide_post_thumbnail_on_restricted_content.php

<?php
/**
 * Do not return the post thumbnail (featured image) on restricted content when viewed by a non-member.
 *
 */
function hide_post_thumbnail_on_restricted_content( $html, $post_id, $post_image_id ) {
	if ( function_exists( 'pmpro_has_membership_access' ) ) {

		// Check if the user has access to the post.
		$hasaccess = pmpro_has_membership_access( $post_id );
	
		if ( empty( $hasaccess ) ) { 
			$html = '';
		}
	}
	return $html;
}
add_filter( 'post_thumbnail_html', 'hide_post_thumbnail_on_restricted_content', 10, 3 );