10 files declare their use of InstallRequirementsInterface

install.inc in core/includes/install.inc
API functions for installing modules and themes.
LayoutDiscoveryRequirements.php in core/modules/layout_discovery/src/Install/Requirements/LayoutDiscoveryRequirements.php
MediaRequirements.php in core/modules/media/src/Install/Requirements/MediaRequirements.php
ModuleInstallRequirements.php in core/modules/system/tests/modules/module_install_requirements/src/Install/Requirements/ModuleInstallRequirements.php
ModuleInstallUnmetRequirementsRequirements.php in core/modules/system/tests/modules/module_install_unmet_requirements/src/Install/Requirements/ModuleInstallUnmetRequirementsRequirements.php
PackageManagerRequirements.php in core/modules/package_manager/src/Install/Requirements/PackageManagerRequirements.php
PgsqlRequirements.php in core/modules/pgsql/src/Install/Requirements/PgsqlRequirements.php
ProfileInstallRequirementsRequirements.php in core/profiles/tests/profile_install_requirements/src/Install/Requirements/ProfileInstallRequirementsRequirements.php
TestingRequirementsRequirements.php in core/profiles/tests/testing_requirements/src/Install/Requirements/TestingRequirementsRequirements.php
WorkspacesRequirements.php in core/modules/workspaces/src/Install/Requirements/WorkspacesRequirements.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.