Minimal PoC demonstrating how symlinks can be used to generate per tag pages in Jekyll.
The directory tag/ contains a file _tag_by_fn.md as well as symlinks to that file. Each symlink is named according to a tag. E.g.:
$ grep bar _posts/*md
_posts/2021-01-04-four.md:tags: bar one
_posts/2021-01-05-five.md:tags: bar two
$ file tag/bar.md
tag/bar.md: symbolic link to _tag_by_fn.md
In _tag_by_fn.md the variable page.name is accessed to determine the set of posts to output.
{% assign output_tag = page.name | remove:'.md' %}
When Jekyll processes symlinks, page.name refers to the name of the symlink, not the name of the target. Example: when tag/bar.md is processed the contents of _tag_by_fn.md are read, but the value of page.name is bar.md. Therefore, a file tag/bar.html will be generated linking all posts tagged bar (i.e., posts four and five).
You can run the following oneliner to automatically generate one symlink for each tag that appears in your posts.
$ grep -Poh '(?<=tags:).*$' _posts/*.md | sed -e 's/\s\+/\n/g' | sort | uniq | xargs -I{} ln -s _tag_by_fn.md tag/{}.md