I tried RepoRecon for a bit:
It indexes quite a lot of KiCad related projects, but I did not see obvious results for “HDI” or for “High Density”.
You can also have a look at the list of: Made with kicad | KiCad Some of those projects are quite densely packed, but I have not looked at details myself. KiCad itself works with nanometer resolution internally, so would be plenty of resolution. But I guess that most of this “High Density” stuff is related to manufacturing itself, and not to the program to design a PCB. And therefore, a general search about managing constraints and design rules for such PCB’s would work. Something like:
https://html.duckduckgo.com/html?q=pcb+hdi+design+rules+constraints