Recently I heard about Fresh and I really liked it, and decided to migrate a small project of mine from Nextjs to Fresh. By the time I got to not found pages I was unable to find any information about them in the Fresh docs. In the docs there is a section called File routing which already shows an example project structure, so to me the most logical place of information about not found or error pages in general was either there or here. It made me think that there were not file based not found pages implemented but then this line in the source code showed me that it was already implemented. The only place with the information about it being existed was in the migration guide. It's worth noting that Fresh 1.x used to have a dedicated "Error pages" section under Concepts.

So I think there still should be a dedicated section for error pages in or even a mention in https://usefresh.dev/docs/advanced/error-handling might help.