To be specific, all sites are processed through Jekyll, unless you add a `.nojekyll` file. Non-Jekyll files (like HTML or CSS) are passed transparently through Jekyll untouched. Adding the dashes to the file header simply tells Jekyll to work its magic.

Yep. The `.nojekyll` option has been around since 2009: https://github.com/blog/572-bypassing-jekyll-on-github-pages

Oh awesome, I didn't realize that everything was going through Jekyll under the hood. Is this new or has it always been like that?

It has always been like that. A file full of static files (no dashes at the top) is still a valid Jekyll site.