Tracking issues are used to facilitate discussion and report on the status of standard library features. All public APIs need a dedicated tracking issue. Some larger internal units of work may also use them.
There's a template that can be used to fill out the initial tracking issue. The Libs team also maintains a Cargo tool that can be used to quickly dump the public API of an unstable feature.
The current state of an unstable feature should be outlined in its tracking issue.
If there's a change you'd like to make to an unstable feature, it can be discussed on the tracking issue first.