Which WordPress backend file?

The domain you mentioned is apparently parked. You are probably concerned that the domain has been canceled, but you are still receiving email notifications from WordPress about plugin updates? If so, the project is still on the original hosting.

How you can deactivate the updates by editing files via FTP is described here, for example: https://www.wpbeginner.com/wp-tutorials/how-to-disable-automatic-updates-in-wordpress/#method-1-disabling-automatic-wordpress-updates-code

However, an alternative would be to first create a backup of the project and then delete all files and the database. As it is not accessible anyway, it does not need to be lying around in the hosting.