Bitgugs is an implementation of an old idea (discussed in Finnish in versionhallintapohjainen tehtävälista). The idea is that the issues of a code (the TODO/bug list) should be versioned with the code, so the natural place to store information about what should be done to the code is in the source code repository of the code. Given this principle, it doesn't much matter in which format we store it there; but version control tools are best suited for dealing with plaintext (raakateksti), so most probably the issues should also be saved in plaintext.
You could do this by hand. There's no problem in having a TODO file in your source code repository, maybe with one issue per line (or paragraph). Or it could be a YAML file. But usually you'll want some helpers for writing and updating those issues. That's where Bitgugs comes in. See https://github.com/pkalliok/bitgugs/blob/main/README.md for details about Bitgugs' design principles.
Bitgugs is a Python script that helps you make neat commits to your issue files with your code changes. It offers facilities for searching issues (e.g. all open issues with a certain word in description), and it will have facilities for managing code reviews as issues.
If you are looking for a simple issue tracker and you have no requirements of non-technical people being able to use it, look at Bitgugs. Bitgugs also frees you from dependencies on third parties, because your TODO metadata will be in your distributed version control in a human readable, machine processable format.