Why UNPKG is free to use, and what will happen if I overuse?

The project is made possible by generous donations from Cloudflare, which serves 99% of our traffic through their world-class CDN, and Google Cloud which donates resources to run our backend infrastructure.

Taken from Patreon

It started out as a fun project, and grew out to a popular service that many people use. Some big companies attached themselves to this project and donate their services and more may do so in the future. They may pull out their donations as their strategies change in the future. For the moment it's free. If the donors pull out it may go down or respond more often with 500 errors or turn into a paid service.

It's not a strictly reliable service as you would get with a paid CDN. But because it's free it's not strictly reliable, if money, donated services or motivation runs out, it might go down overnight. Good for hobby or low traffic projects, but don't depend on it for mission critical things according to this blog.

https://kentcdodds.com/blog/unpkg-an-open-source-cdn-for-npm

unpkg is great for open source project demos and instructional material (I use it heavily in my Beginner's Guide to ReactJS), but it's not well suited for mission-critical applications at scale because:

unpkg is a free, best-effort service and cannot provide any uptime or support guarantees.

That's why Michael recommends:

if you rely on it to serve files that are crucial to your business, you should probably pay for a host with well-supported infrastructure and uptime guarantees.