Using https://www.11ty.dev/docs/languages/custom/#aliasing-an-existing-template-language

eleventyConfig.addExtension("markdown", {
	key: "md",
});

Any files with the .markdown extension were previously included if md was included in your project template format keys. As we unlock the power to more complex aliases in 3.0, this is changing.

You will now need to explicitly add markdown in --formats=markdown or via eleventyConfig.addTemplateFormats("markdown") to process .markdown files in your project.

This was more important as more and things chain to the 11ty.js template type (11ty.tsx, 11ty.jsx, 11ty.ts, etc).