Does Sveltekit have a preview mode?

Nothing built in as such. But the approach would be exactly the same as in those frameworks: instead of going to /posts/my-new-article you make it such that /posts/my-new-article?preview will also get posts that are in preview only.

To get the query parameters you can use

export async function get({ query }) {
  const preview = query.get('preview')
}