Register a Custom Taxonomy

<?php
add_action( 'init', 'scseed_register_topic_taxonomy' );
if ( ! function_exists( 'scseed_register_topic_taxonomy' ) ) {
    function scseed_register_topic_taxonomy() {
        $labels = array(
            'name'              => 'Topics',
            'singular_name'     => 'Topic',
            'search_items'      => 'Search Topics',
            'all_items'         => 'All Topics',
            'parent_item'       => 'Parent Topic',
            'parent_item_colon' => 'Parent Topic:',
            'edit_item'         => 'Edit Topic',
            'update_item'       => 'Update Topic',
            'add_new_item'      => 'Add New Topic',
            'new_item_name'     => 'New Topic Name',
            'menu_name'         => 'Topics',
        );
        register_taxonomy( 'topic', array( 'post' ), array(
            'labels'            => $labels,
            'hierarchical'      => true,
            'public'            => true,
            'rewrite'           => array( 'slug' => 'topic' ),
            'show_admin_column' => true,
            'show_in_rest'      => true,
        ) );
    }
}