6 files declare their use of InstantArticleFormatterInterface in Facebook Instant Articles 3.x

AuthorReferenceFormatter.php in src/Plugin/Field/FieldFormatter/AuthorReferenceFormatter.php

EntityReferenceEntityFormatter.php in src/Plugin/Field/FieldFormatter/EntityReferenceEntityFormatter.php

FieldItemListNormalizer.php in src/Normalizer/FieldItemListNormalizer.php

FormatterBase.php in src/Plugin/Field/FieldFormatter/FormatterBase.php

ImageFormatter.php in src/Plugin/Field/FieldFormatter/ImageFormatter.php

VideoFormatter.php in src/Plugin/Field/FieldFormatter/VideoFormatter.php