Why even write one of those? Just set up an nginx proxy cache and call it done.
But that doesn't even scratch the surface of what this project does. We already have a few simple approaches to this problem: apt-cacher and apt-cacher-ng. Neither of which were any help when Canonical was under DDoS. Both of which have proven unreliable.
As I said elsewhere, apt-cacher-ultra is a pretty subtle piece of software, it doesn't just do a naive cache, it fully understands the apt repo structure, validates the repo, and snapshots it, understands what packages your clients frequently request and preemptively downloads updates to them as part of the snapshot promotion, etc. A DDoS of the package sources, even in the middle of an upstream releasing updates, is something a-c-u will handle no problem. I've simulated this by firewall rules blocking upstream traffic. It's also proven much more reliable than apt-cacher-ng.
I've been in the software game ~40 years, I've seen it time and time again: "How hard could it be" and then when you get into it you realize there are a lot of devils in the details.