6 files declare their use of MarkdownFilterInterface in Markdown 3.0.x

AtAutolinker.php in src/Plugin/Markdown/Extension/AtAutolinker.php

BaseExtension.php in src/Plugin/Markdown/Extension/BaseExtension.php

HashAutolinker.php in src/Plugin/Markdown/Extension/HashAutolinker.php

LinkRenderer.php in src/Plugin/Markdown/Extension/LinkRenderer.php

MarkdownExtensionInterface.php in src/Plugin/Markdown/Extension/MarkdownExtensionInterface.php

MarkdownParserManager.php in src/MarkdownParserManager.php