I should write a blog post about it. It has steadily gained more traffic over the years and hasn't need much care and feeding.
The go code can be found here if you're interested, though it some of the ugliest code I've written. I made it when ifconfig.me was having load issues many years ago.
I do have the service behind CloudFlare, which is essentially a reverse proxy. The reason is for CloudFlare is non-obvious; it is deserves a blog post, and that is connection pooling.
If I have all the requests go back to the origin, the bottle neck is not the go code, but Linux opening and closing all those single use TCP sessions. CloudFlare creates around 100k persistent connections to the backend, but then just keeps them open. This makes Linux much happier.
The high connection count actually made the service unstable once: https://github.com/georgyo/ifconfig.io/issues/2
