If you are using SlimSeo plugin it automatically add all posts and taxonomies into sitemap.xml file. This snippet allows you to exclude some post types and taxonomies from sitemap file.
add_filter( 'slim_seo_sitemap_post_types', function( $post_types ) {
$post_types = array_diff( $post_types, ['help','slider'] );
return $post_types;
} );
add_filter( 'slim_seo_sitemap_taxonomies', function( $taxonomies ) {
$taxonomies = array_diff( $taxonomies, ['category'] );
return $taxonomies;
} );
In this snippet above from sitemaps file will exclude ‘help’ and ‘slider’ post type, and ‘category’ taxonomy.