In TSPL Book, I don't see any subsections listed under chapters in the table of contents. This makes it really hard to navigate the document.
Is this something that's going to be added later?
Oops! I am sorry, Workgroup Updates is the wrong place for this.
Is there a place where feedback can be provided?
Since TSPL is now open source, you can submit issues and feature requests directly to the project here:
Yep, like James says, an issue in the book's repo is a good place to start.
In one of the docs workgroup meetings (which you're welcome to join by the way) I believe someone also brought up the idea that DocC's quick navigation and potentially the sidebar should have a publication-time option to include the subsections you mentioned.
The data's certainly there… it shows up in the right hand side of each chapter if your viewport is wide enough, but I can see how it'd be nice to scan some of those in a more global fashion earlier on.