You can use the filters the_content_feed
and the_excerpt_rss
to decode & strip 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' );