6 files declare their use of OutputElementInterface in Extensible BBCode 4.0.x

PreparedTagElement.php in src/PreparedTagElement.php

TagPluginBase.php in src/Plugin/TagPluginBase.php

TagPluginInterface.php in src/Plugin/TagPluginInterface.php

TagProcessorBase.php in src/Parser/Processor/TagProcessorBase.php

TagProcessorInterface.php in src/Parser/Processor/TagProcessorInterface.php

TagProcessResult.php in src/TagProcessResult.php