6 files declare their use of UpdateFetcherInterface in Drupal 8

update.api.php in core/modules/update/update.api.php

Hooks provided by the Update Manager module.

update.compare.inc in core/modules/update/update.compare.inc

Code required only when comparing available updates to existing data.

update.install in core/modules/update/update.install

Install, update, and uninstall functions for the Update Manager module.

update.module in core/modules/update/update.module

Handles updates of Drupal core and contributed projects.

update.report.inc in core/modules/update/update.report.inc

Code required only when rendering the available updates report.

UpdateManagerUpdate.php in core/modules/update/src/Form/UpdateManagerUpdate.php