Hey @Larandar. This is documented on the Jekyll page here: https://jekyllrb.com/docs/structure/ in this section:

Every file or directory beginning with the following characters: . , _ , # or ~ in the source directory will not be included in the destination folder. Such paths will have to be explicitly specified via the config file in the include directive to make sure they’re copied over

I agree this is a bit buried. If you have any feedback on places you expected this to be documented, I can see about adding it so it’s easier to find next time. Thanks posting a link to an answer that helped!

I guess what induced me in error is the first paragraph of the documentation :

GitHub Pages is a static site hosting service that takes HTML, CSS, and JavaScript files straight from a repository on GitHub, optionally runs the files through a build process, and publishes a website. You can see examples of GitHub Pages sites in the GitHub Pages examples collection.

It says it optionally build them when in fact it’s always built using Jekyll unless you put a .nojekyll file

Ah yeh that is confusing. Technically if you just give us html/css, there’s no build step, but Jekyll is still what determines whether or not to do the build and it will ignore folders with _ prefixes by default, so you have to explicitly tell Jekyll to scan those folders. We’re working on making changes here, but if you’re not using Jekyll I highly recommend looking at some of the GitHub Actions that do the build/deploy for you. These are typically setup to do the .nojekyll bit for you as well as doing a build with your tool of choice. We’ll work on clarifying the docs as well.