How do I disable the Update manager module?
You cannot disable a module as you do with Drupal 7 or previous versions, and this is valid also for the Update manager module. You could eventually avoid the module checks for new module versions, but the module itself doesn't have a setting for that in admin/reports/updates/settings.


  


What you could do is, using a custom module, avoiding Drupal invokes update_cron() which is the hook that periodically checks for updates.

function mymodule_implements_alter(&amp;$implementations, $hook) {
  if ($hook == 'cron') {
    unset($implementations['update']);
  }
}


You could still manually check for updates from admin/reports/updates, but Drupal would not check for updates during cron tasks.

If then you also need to disable the route set by the Update manager module, you could also do it as shown in How do I alter the route defined by another module?
This is, I think, the closer you could get to disable the Update manager module.