Serve a subset of terms at /term-slug instead of /tag/term-slug

gistfile1.php

<?php

/**
 * 'special-term' is a unique butterfly
 */
function dbx_get_special_terms() {
	return array( 'special-term' );
}

/**
 * Update our special terms' term links
 */
add_filter( 'term_link', function( $termlink, $term, $taxonomy ) {
	global $wp_rewrite;

	if ( 'post_tag' !== $taxonomy ) {
		return $termlink;
	}

	if ( in_array( $term->slug, dbx_get_special_terms() ) ) {
		$tag_base = get_option( 'tag_base' );
		if ( empty( $tag_base ) ) {
			$tag_base = 'tag';
		}
		$termlink = str_replace( "/{$tag_base}/", "/", $termlink );
	}

	return $termlink;
}, 10, 3 );

/**
 * Custom URI structures need custom rewrite rules
 */
add_action( 'init', function() {

	$special_terms_string = implode( '|', dbx_get_special_terms() );

	add_rewrite_rule( '^(' . $special_terms_string . ')/feed/(feed|rdf|rss|rss2|atom)/?$', 'index.php?tag=$matches[1]&feed=$matches[2]', 'top' );
	add_rewrite_rule( '^(' . $special_terms_string . ')/(feed|rdf|rss|rss2|atom)/?$', 'index.php?tag=$matches[1]&feed=$matches[2]', 'top' );
	add_rewrite_rule( '^(' . $special_terms_string . ')/page/?([0-9]{1,})/?$', 'index.php?tag=$matches[1]&paged=$matches[2]', 'top' );
	add_rewrite_rule( '^(' . $special_terms_string . ')/?$', 'index.php?tag=$matches[1]', 'top' );

});

/**
 * Canonical redirect for our special terms
 * Resolves /tag/special-terms/ to /special-terms/
 */
add_action( 'template_redirect', function() {

	if ( ! is_tag() ) {
		return;
	}

	if ( ! in_array( get_queried_object()->slug, dbx_get_special_terms() ) ) {
		return;
	}

	$tag_base = get_option( 'tag_base' );
	if ( empty( $tag_base ) ) {
		$tag_base = 'tag';
	}
	if ( 0 !== stripos( $_SERVER['REQUEST_URI'], '/tag/' ) ) {
		return;
	}
	wp_safe_redirect( get_term_link( get_queried_object() ) );
	exit;
});