I think sadly you won’t be able to read the preeminent examples of really old antifragile code that’s still powering the world. That stuff is mostly proprietary - banks and governments and so on. Hence this article.
BUT - there are still plenty of mature open source codebases to read, especially operating systems, databases, and Unix utilities. Sure, they’re living projects that now look quite different from the original, but they’re descended from code a few decades old and written in the same language — C. There’s Emacs (35 yrs old), GCC (33 yrs), the Linux kernel (29 yrs), MySQL (25 yrs), and Postgres (a youngster, only 24).
And then there’s the programming languages. C itself is now 48 years old! Since it’s the language of the Linux Kernel and Postgres and so much else, I’d be willing to bet it’ll be around for another 48 years. (Yes, I love Rust, and I’d use it over C for any new project, but this is the Linux Kernel folks!)
But - since this is Hacker News - I should note that C is beaten by a long long way by Lisp, which is 62 years old. That’s older than COBOL - and people still love it. John McCarthy really had programming figured out.