PostCSS 8 was released. It added a new plugin API. Right now Gatsby does not support PostCSS 8 plugins.
Updating postcss-loader and adding postcss to dependencies for gatsby-plugin-postcss will fix the problem.
PostCSS 8 changelog does not have breaking changes related to Gatsby only removing Node.js 6 and 8 support).
We'd be happy to receive a PR updating this, if you need help feel free to reach out here.
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