您可以使用过滤器the_content_feed 和the_excerpt_rss 解码（&A）；带HTML：

function wpse_311515_plain_text( $content ) {
    $content = wp_strip_all_tags( $content );
    $content = html_entity_decode( $content, ENT_QUOTES, get_bloginfo( \'charset\' ) );

    return $content;
}

add_filter( \'the_content_feed\', \'wpse_311515_plain_text\' );
add_filter( \'the_excerpt_rss\',  \'wpse_311515_plain_text\' );