How can I provide a plaintext RSS feed?

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' );