The Squid codebase had a custom malloc in it for a while for some operating systems, but on systems with competent malloc it used the OS implementation. I doubt that malloc remains today (I believe Squid has been somewhat rewritten in C++, with a more modern approach to memory allocation, since I was working on it).

There was a time about 15 years ago, when many UNIX variants had sub-optimal malloc, or other core libraries, and custom implementations were common in large Open Source projects (as well as commercial products). Those operating systems are mostly gone (Solaris remains, kinda, and there's still some big iron running those old beasts), but I haven't seen a newish project using a custom malloc or other core library in years. I couldn't name the projects that made that choice back then, but I remember seeing it very frequently, when I used to read a lot more C source than I do, today.