Taxonomy base class. Uses extended-cpts (https://github.com/johnbillion/extended-cpts) by johnbillion (https://github.com/johnbillion).

class-taxonomy-base.php

<?php
/**
 * Encompass Health Blog Taxonomy Base Class
 *
 * Base class for creating and modifying all taxonomies for Encompass Health Blog.
 *
 * @package EH\Taxonomies
 * @author  Chris Reynolds <chris@humanmade.com>
 */

namespace EH\Taxonomies;

/**
 * Base class for registering or modifying Encompass Health Blog taxonomies.
 */
class Taxonomy_Base {

	/**
	 * Init function used to load in any hooks in child classes.
	 */
	public function init() {}

	/**
	 * The taxonomy name.
	 *
	 * @return string Returns the taxonomy name.
	 */
	public function taxonomy_name() {
		return '';
	}

	/**
	 * The terms for a taxonomy.
	 *
	 * @return array Associative array of taxonomy terms and arguments.
	 */
	public function terms() {
		return [];
	}

	/**
	 * The allowed post types for the taxonomy, if we're registering it.
	 *
	 * @return array An array of post types.
	 */
	public function post_types() {
		return [ 'post' ];
	}

	/**
	 * An array of arguments to pass to Extended CPTs when registering the taxonomy.
	 *
	 * @return array An array of allowed arguments.
	 */
	public function taxonomy_args() {
		return [];
	}

	/**
	 * An array of labels (singular, plural and slug) used to register the taxonomy.
	 *
	 * @return array An array of labels.
	 */
	public function taxonomy_labels() {
		return [];
	}

	/**
	 * Register the taxonomy if it doesn't already exist.
	 */
	public function register() {
		if ( ! taxonomy_exists( $this->taxonomy_name() ) ) {
			register_extended_taxonomy( $this->taxonomy_name(), $this->post_types(), $this->taxonomy_args(), $this->taxonomy_labels() );
		}
	}

	/**
	 * Insert the terms for the taxonomy.
	 * Allows for child terms to be added if structured inside a 'children' parameter in the array of terms.
	 */
	public function init_terms() {
		foreach ( $this->terms() as $term_name => $args ) {
			$term = wp_insert_term( $term_name, $this->taxonomy_name(), $args );

			// Report if there's an error.
			if ( is_wp_error( $term ) ) {
				// translators: 1: the term, 2: the error message.
				wp_die( esc_html( sprintf( __( 'Could not insert term %1$s. The error returned was: %2$s', 'encompass' ), $term_name, $term->get_error_message() ) ), esc_html__( 'Error inserting term.', 'encompass' ) );
			}

			// If child terms were passed to this term, loop through those and add them, too.
			if ( isset( $args['children'] ) && is_array( $args['children'] ) ) {
				foreach ( $args['children'] as $child_term_name => $child_term_args ) {
					$child_term_args = array_merge( $child_term_args, [ 'parent' => $term['term_taxonomy_id'] ] );
					wp_insert_term( $child_term_name, $this->taxonomy_name(), $child_term_args );
				}
			}
		}
	}

	/**
	 * Removes the taxonomy page if not needed.
	 */
	public function remove_menu_page() {
		remove_submenu_page( 'edit.php', sprintf( 'edit-tags.php?taxonomy=%s', $this->taxonomy_name() ) );
	}
}

namespace.php

<?php
/**
 * Example bootstrap.
 */

namespace Taxonomies;

function bootstrap() {
	$example_tax = new ExampleTax();
	$example_tax->init();
}

taxonomy-example.php

<?php
/**
 * Example taxonomy
 *
 * Creates the an example taxonomy and handles related functionality.
 *
 * @package Taxonomies
 * @author  Chris Reynolds <chris@humanmade.com>
 */

namespace Taxonomies;

/**
 * Example Taxonomy taxonomy class.
 */
class ExampleTax extends Taxonomy_Base {
	const TAXONOMY_NAME = 'example_taxo';

	/**
	 * Handle all the Audience Types hooks.
	 */
	public function init() {
  		add_action( 'init', [ $this, 'register' ], 10 );
		add_action( 'init', [ $this, 'init_terms' ], 20 );
		add_action( 'admin_menu', [ $this, 'remove_menu_page' ] );
	}

	/**
	 * Returns the taxonomy name.
	 *
	 * @return string The taxonomy name.
	 */
	public function taxonomy_name() {
		return self::TAXONOMY_NAME;
	}

	/**
	 * The list of audience types.
	 *
	 * @return array Array of taxonomy terms and arguments.
	 */
	public function terms() {
		return [
			__( 'Apples', 'textdomain' ) => [
				// Optional taxonomy term arguments go here.
			],
			__( 'Oranges', 'textdomain' ) => [
				// Optional taxonomy term arguments go here.
			],
			__( 'Avocados', 'textdomain' ) => [
				// Optional taxonomy term arguments go here.
			],
			__( 'Bananas', 'textdomain' ) => [
				// Optional taxonomy term arguments go here.
			],
		];
	}

	/**
	 * The list of supported post types.
	 *
	 * @return array An array of post types supported by the tax.
	 */
	public function post_types() {
		return [ 'post' ];
	}

	/**
	 * Defines the labels used in registering the taxonomy.
	 *
	 * @return array An array of audience type labels.
	 */
	public function taxonomy_labels() {
		return [
			'singular' => __( 'Example', 'textdomain' ),
			'plural'   => __( 'Examples', 'textdomain' ),
			'slug'     => 'example',
		];
	}

	/**
	 * Any allowed arguments for extended taxonomies.
	 * 
	 * @see https://github.com/johnbillion/extended-cpts/wiki/Registering-taxonomies
	 * @return array An array of taxonomy arguments.
	 */
  	public function taxonomy_args() {
  		return [ 'meta_box' => 'radio' ];
  	}
}