6 classes are annotated with ActivityDestination in Open Social 10.3.x

EmailActivityDestination in modules/custom/activity_send/modules/activity_send_email/src/Plugin/ActivityDestination/EmailActivityDestination.php

Provides a 'EmailActivityDestination' activity destination.

NotificationsActivityDestination in modules/custom/activity_basics/src/Plugin/ActivityDestination/NotificationsActivityDestination.php

Provides a 'NotificationsActivityDestination' acitivy destination.

StreamExploreActivityDestination in modules/custom/activity_basics/src/Plugin/ActivityDestination/StreamExploreActivityDestination.php

Provides a 'StreamExploreActivityDestination' acitivy destination.

StreamGroupActivityDestination in modules/custom/activity_basics/src/Plugin/ActivityDestination/StreamGroupActivityDestination.php

Provides a 'StreamGroupActivityDestination' acitivy destination.

StreamHomeActivityDestination in modules/custom/activity_basics/src/Plugin/ActivityDestination/StreamHomeActivityDestination.php

Provides a 'StreamHomeActivityDestination' acitivy destination.

StreamProfileActivityDestination in modules/custom/activity_basics/src/Plugin/ActivityDestination/StreamProfileActivityDestination.php

Provides a 'StreamProfileActivityDestination' acitivy destination.