Remove /category/ From Category URLs

add_filter( 'category_link', function ( $link ) {
    $base = trim( get_option( 'category_base' ) ? get_option( 'category_base' ) : 'category', '/' );

    return str_replace( home_url( '/' . $base . '/' ), home_url( '/' ), $link );
} );

add_filter( 'category_rewrite_rules', function () {
    $rules = array();

    foreach ( get_categories( array( 'hide_empty' => false ) ) as $category ) {
        $path = $category->parent ? get_category_parents( $category->term_id, false, '/', true ) : $category->slug . '/';

        if ( is_wp_error( $path ) ) {
            continue;
        }

        $path = preg_quote( trim( $path, '/' ), '#' );
        $rules[ "($path)/(?:feed/)?(feed|rdf|rss|rss2|atom)/?$" ] = 'index.php?category_name=$matches[1]&feed=$matches[2]';
        $rules[ "($path)/page/?([0-9]{1,})/?$" ]                  = 'index.php?category_name=$matches[1]&paged=$matches[2]';
        $rules[ "($path)/?$" ]                                    = 'index.php?category_name=$matches[1]';
    }

    // Old /category/... addresses are caught and redirected below.
    $base = trim( get_option( 'category_base' ) ? get_option( 'category_base' ) : 'category', '/' );
    $rules[ preg_quote( $base, '#' ) . '/(.+?)/?$' ] = 'index.php?wpide_old_category=$matches[1]';

    return $rules;
} );

add_filter( 'query_vars', function ( $vars ) {
    $vars[] = 'wpide_old_category';

    return $vars;
} );

add_action( 'template_redirect', function () {
    $old = get_query_var( 'wpide_old_category' );

    if ( $old ) {
        wp_safe_redirect( home_url( '/' . trim( sanitize_text_field( $old ), '/' ) . '/' ), 301 );
        exit;
    }
} );

// Rebuild the rules when categories change.
foreach ( array( 'created_category', 'edited_category', 'delete_category' ) as $wpide_cat_hook ) {
    add_action( $wpide_cat_hook, function () {
        delete_option( 'rewrite_rules' );
    } );
}