Researching this for some hours I want to document it:
Collections without frontmatter are not possible
More exactly: adding Markdown files without frontmatter in a collection will not work as expected. The Markdown file will simply be published as is (no HTML) to your site.
The findings:
- Worked in very old Jekyll versions (2.x). Broke around v3.1 and wasn’t fixed, but instead documented.
- Not even supported by the jekyll-optional-front-matter plugin
- Using plugin jekyll-title-from-headings also doesn’t help
Conclusion
So if you have to work with input files without front matter you have to treat them as posts or pages to avoid Jekyll just copying the Markdown to the target directory.
Workaround using subdir+category
If you have a use case where you did rely on the collection list being available under site.<collection> you could iterate over all posts/pages of a common category instead.
If you keep all the documents in a subdir setting the category by default from matter can help:
defaults:
- scope:
path: subdir
values:
category: name-of-collection