I wrote a Mach-O parser for Homebrew (the macOS package manager) that’s invoked on just about every package install, so that’s probably a couple of million daily users. It also ended up as a dependency of CocoaPods at some point, so it’s on the critical path for a good chunk of the App ecosystem as well.

I also implemented 2FA and API tokens for PyPI (and helped/continue to work on lots of other parts of that ecosystem).