I can answer this a bit - I maintain Yarn, a JS package manager. While nowhere near Curl, it does have a fairly significant user base - at least a few million devs, from what I understand. For all this work, I received exactly two emails thanking me for my work during the past four years.

I also happen to maintain a little "Secret Santa" website, hosted on a GitHub page. Nothing too fancy, just a static app that lets you manage a Secret Santa without creating accounts. Well, every year, I receive 3-4 emails thanking me for creating it, which is even more surprising considering they often come from people that aren't from the tech world at all.

Perhaps for my happiness I should invest more in this side project than in a package manager used across the globe :)