I'm going to participate in a debate on monolithic vs many smaller repositories.

One of my points is that many companies does not have a scale where investing heavily in custom tooling has a feasible ROI.

I know that Facebook builds and ships the very newest Mercurial with extensions. https://www.youtube.com/watch?v=gOVD-DrUpwQ&t=169s

Uber uses ( or has used ) a custom build of Docker to do some scrubbing of build-time secrets.

What other companies do you know of that uses custom built versions of common tools like Git? And for what purpose?

I look forward to seeing some cool examples of custom tooling.