7 files declare their use of PermissionProviderInterface in Group 2.0.x

AccessControlTest.php in tests/src/Unit/AccessControlTest.php

BarAdminPermissionProvider.php in tests/modules/group_test_plugin_alter/src/Plugin/Group/RelationHandler/BarAdminPermissionProvider.php

BazAdminPermissionProvider.php in tests/modules/group_test_plugin_alter/src/Plugin/Group/RelationHandler/BazAdminPermissionProvider.php

FooAdminPermissionProvider.php in tests/modules/group_test_plugin_alter/src/Plugin/Group/RelationHandler/FooAdminPermissionProvider.php

FullEntityPermissionProvider.php in tests/modules/group_test_plugin/src/Plugin/Group/RelationHandler/FullEntityPermissionProvider.php

GroupNodePermissionProvider.php in modules/gnode/src/Plugin/Group/RelationHandler/GroupNodePermissionProvider.php

PermissionProvider.php in src/Plugin/Group/RelationHandlerDefault/PermissionProvider.php