This scenario may occur once our SharePoint site is able to be migrated to another site or it may be all the content might be migrated and all the users are familiar with the old site URL and not aware of the newly migrated SharePoint site.
In that case we can make use of this redirect option which will redirect the user to the new site whenever the user tries to access the old SharePoint site/page.
This can be done through the Master Page. Below are the steps,
  1. Go to Site Actions --> Site Settings --> Click Master pages and page layouts under Web Designer Galleries option.

    NOTE :

    If Master pages and page layouts option is not available then please enable Server Publishing feature both in Site and Site Collection level.

  2. Select the Default Master page which is enabled currently in your site. for E.G. "seattle.master".

  3. Download the copy of the Master page and just make a back up before editing.

  4. Once Back up is done try to edit the master page file. Try to edit in NotePad.

  5. In the Master Page files find where the META tag starts add the below line before the META tag,
    1. <meta http-equiv="Refresh" content="1; url=www.google.com">
  6. Once done save the file with the same name and extension and then upload it in the "Master Page and Page Layouts" gallery.

  7. After uploading the master page make sure it is approved and make it as a current master page. that's it we are done. so whenever a user tries to access your existing SharePoint page/Site he/she will be automatically redirected to google landing page.