my_module.token.inc

<?php

use Drupal\Core\Render\BubbleableMetadata;

/**
 * @implements hook_token_info().
 */
function my_module_token_info() {
  $type = [
    'name' => t('Custom tokens'),
    'description' => t('My custom tokens.'),
  ];

  $customTokens['my_string'] = [
    'name' => t("My string"),
    'description' => t('My string to be used.'),
  ];

  return [
    'types' => [
      'customTokens' => $type
    ],
    'tokens' => [
      'customTokens' => $customTokens
    ],
  ];
}

/**
 * @implements hook_tokens().
 *
 * @param string $type
 * @param array $tokens
 * @param array $data
 * @param array $options
 * @param BubbleableMetadata $bubbleable_metadata
 *
 * @return array $replacements
 */
function my_module_tokens($type, $tokens, array $data, array $options, BubbleableMetadata $bubbleable_metadata) {
  $replacements = [];

  if ($type == 'customTokens') {
    foreach ($tokens as $name => $original) {
      switch ($name) {
        case 'my_string':
          $replacements[$original] = 'my custom string';
          break;
      }
    }
  }

  return $replacements;
}