Grav CMS is a flat-file lightweight open-source content management system. It’s designed to be easy to set up and learn. It focuses on speed and simplicity rather than providing complex built-in features.

Pico CMS is a simple lightweight open-source and flat-file content management system with no administration back-end and database to deal with. It’s a PHP-based content management system that converts .txt files into pages.

WonderCMS is a small flat-file and open-source CMS with fast & easy editing capabilities. It uses a text file as a database and being installed without any configurations, since no relational database is required.

Automad CMS is a flat-file open-source content management system and template engine. It’s fast and lightweight cms that stores content in a human readable form. The file structure of the pages directory reflects the data structure of the website.

Batflat CMS is a bootstrap ready lightweight CMS that uses flat-files for saving data. It’s created as a lightweight alternative to heavy and complex CMS systems.

TinyCMS is an extremely simple content management system. It’s an easy to use, robust and lightweight tool to build websites. It has a flat-file database which makes it easy to install. It uses modules to add additional features. 

Pulse CMS is a simple software for small websites. It enables web developers to add web content management system capabilities to an existing site.

Here is a list of other cms (free and paid) platforms without database to be considered:

Kirby CMS(https://getkirby.com/)

Bolt CMS(https://boltcms.io/)

Monstra(https://monstra.org/)

GetSimple(http://get-simple.info/)

HTMLy(https://www.htmly.com/)

Typesetter(https://www.typesettercms.com/)

FlexType(https://flextype.org/)

Statamic(https://statamic.com/)

Bludit(https://www.bludit.com/)

PhileCMS(https://philecms.github.io/)

NestaCMS(http://nestacms.com/)

Baun(https://bauncms.com/)

Mecha(https://mecha-cms.com/)

Flatpress(https://www.flatpress.org/)

CMSimple(https://www.cmsimple.org/en/)

Sphido(https://sphido.org/)

Source: https://technocript.com/cms-without-database/