Grav is an easy to use, yet powerful, opensource CMS that requires no database. In a nutshell, installing Grav is a matter of unzipping the file. It is fast, has sophisticated caching, and a light footprint for optimal performance.
ClassicPress is a community-led open source content management system and a fork of WordPress that preserves the TinyMCE classic editor as the default option, which means there is no Gutenberg (block) editor.
Bludit is a web application to build your own website or blog in seconds, it's completely free and open source. A PRO version is available for those who want to support the software developer. The software seems almost the same between the versions.
Automad is a file-based flat CMS and template engine. It is designed to be fully portable without any dependencies on databases or fixed locations and doesn't require any complicated setup process. All data gets stored in a human readable form.
FlatPress is an open-source standard-compliant multi-lingual extensible blogging engine which does not require a DataBase Management System to work. You don’t need MySQL because FlatPress stores all of its content on text files.
Vapid is an intentionally simple content management system built on the idea that you can create a custom dashboard without ever leaving the HTML.Discontinued
Pulse CMS is the easiest way to build and deploy a responsive, content managed website. Since it's a flat file CMS there is no complicated database setup, just copy it to your server and go.