So far our peerDependencies made it seem like we're actively supporting all versions of Vite and MDX, so in this release we're locking down the supported versions of MDX and Vite to manage expectations of this plugin, because we're not actively maintaining it.
You may be able to get it working with MDX v2 and Vite v3+, but this change is intended to communicate that for up-to-date support we're recommending MDX's official Rollup plugin, @mdx-js/rollup instead of this Vite plugin, for the time being.