7 files declare their use of FieldGroupFormatterBase in Field Group 8

Accordion.php in src/Plugin/field_group/FieldGroupFormatter/Accordion.php

AccordionItem.php in src/Plugin/field_group/FieldGroupFormatter/AccordionItem.php

Details.php in src/Plugin/field_group/FieldGroupFormatter/Details.php

Fieldset.php in src/Plugin/field_group/FieldGroupFormatter/Fieldset.php

HtmlElement.php in src/Plugin/field_group/FieldGroupFormatter/HtmlElement.php

Tab.php in src/Plugin/field_group/FieldGroupFormatter/Tab.php

Tabs.php in src/Plugin/field_group/FieldGroupFormatter/Tabs.php