I'm curious to why the polyfill example uses unpkg.com. It is quite unreliable and has broken sites many times.
jsdelivr.com is much more reliable (Multi-CDN, Multi-DNS). Comparison: https://www.jsdelivr.com/unpkg
I am not affiliated in anyway to jsdeliver or unpkg. I simply used to be a user on unpkg.
Maybe for tracking purposes, because most google-affiliated CDNs are widely blocked and unpkg might be small enough to not immediately raise eyebrows with devs. Unpkg removed their SPONSORS.md file recently, and their README claims to be hosted at fly.io which seems to be behind Cloudflare.