WooCommerce Custom Rest API Endpoint v3 [creating for https://domain.tld/wc-api/v3/my-custom-endpoint]

plugin-main-file.php

add_action( 'woocommerce_api_loaded', 'my_plugin_load_api' );
add_filter( 'woocommerce_api_classes', 'my_plugin_add_api' );

function my_plugin_load_api() {
    include_once 'your-api-endpoint-class.php';
}

function my_plugin_add_api( $apis ) {
    $apis[] = 'YOUR_API_ENDPOINT_CLASS_NAME';
    return $apis;
}

your-api-endpoint-class.php

class YOUR_API_ENDPOINT_CLASS_NAME extends WC_API_Resource {
     protected $base = '/my-custom-endpoint';

	public function register_routes( $routes ) {
		$routes[ $this->base ] = array(
			array( array( $this, 'get_custom' ), WC_API_Server::READABLE )
		);
		return $routes;
	}
	
	public function get_custom() {
		return array( 'custom' => 'Data' );
	}
}