Example of hook_uninstall() for migration-related module.

pix_migrate.install.php

<?php

/**
 * @file
 * Pix migration install file.
 */

/**
 * Implements hook_uninstall().
 */
function pix_migrate_uninstall() {
  // Delete this module's migrations.
  $migrations = [
    'pix_file',
    'pix_gallery',
    'pix_image',
    'pix_name',
    'pix_user',
  ];
  foreach ($migrations as $migration) {
    Drupal::configFactory()->getEditable('migrate_plus.migration.' . $migration)->delete();
  }
}