7 files declare their use of DashboardBlockBase in Content Planner 8

ContentStateStatistic.php in modules/content_kanban/src/Plugin/DashboardBlock/ContentStateStatistic.php

CustomHTMLBlockBase.php in src/Plugin/DashboardBlock/CustomHTMLBlockBase.php

DashboardSettingsForm.php in src/Form/DashboardSettingsForm.php

RecentCalendarPublicationsBlock.php in modules/content_calendar/src/Plugin/DashboardBlock/RecentCalendarPublicationsBlock.php

RecentKanbanActivities.php in modules/content_kanban/src/Plugin/DashboardBlock/RecentKanbanActivities.php

UserBlock.php in src/Plugin/DashboardBlock/UserBlock.php

ViewBlockBase.php in src/Plugin/DashboardBlock/ViewBlockBase.php