Automad is a open source flat file CMS.
The Simplest Content Management System. Ever.
Grav is a modern open source flat-file CMS
HTMLy is an open source databaseless blogging platform. The Flat-File Blog and Flat-File CMS written in PHP.
Kirby is a file‑based CMS Easy to setup. Easy to use. Flexible as hell.
PhileCMS is a file-driven CMS. Powered By Markdown And Twig. But You Can Change That.
Pico is a stupidly simple & blazing fast, flat file CMS.
Sphido is a rocket fast, lightweight, flat file CMS for PHP
Stacey is a lightweight content management system.