6 files declare their use of ActivityType in CRM Core 8.3

ActivityAccessControlHandler.php in modules/crm_core_activity/src/ActivityAccessControlHandler.php

ActivityUiTest.php in modules/crm_core_activity/tests/src/Functional/ActivityUiTest.php

CrmCoreDemoTest.php in modules/crm_core_demo/tests/src/Functional/CrmCoreDemoTest.php

CRMFeedsActivityProcessor.inc in modules/crm_core_activity/legacy/CRMFeedsActivityProcessor.inc

Class definition of CRMFeedsActivityProcessor. @TODO: Remove Feeds integration

IndividualCRUDTest.php in modules/crm_core_contact/tests/src/Kernel/IndividualCRUDTest.php

OrganizationCRUDTest.php in modules/crm_core_contact/tests/src/Kernel/OrganizationCRUDTest.php