This is pretty neat stuff. I wonder if it takes a corporation working on closed-source code to implement stuff like this, i.e., work that is generally unglamorous, tedious, and does not have enough open-source developers who would see a personal benefit. My closest guess for an analogous open-source endeavor would be projects to improve Linux font rendering, maybe Wayland, or perhaps systemd, although all of those are still many times more ‘mainstream’ than supporting a script like Soyombo, whose use according to Wikipedia is “practically nonexistent today.”
If that were the case, HarfBuzz wouldn’t exist. Engineers love challenging puzzles, and shaping and layout are just really sticky puzzles with tons of parameters.