Polyfill.io was owned by the Financial Times web team, then moved under community management, and the last maintainer sold the project to a weird Chinese CDN company, and they moved it away from Fastly (the CDN / Edge compute platform running the OSS code for the service) and started to mess with the returned files.

From polyfill.io original creator: twitter.com/triblondon/status/1761852117579427975

Issue on their repo: polyfillpolyfill/polyfill-service/issues/2834

The domain polyfill.io is now a CNAME to polyfill.io.bsclink.cn, there has been no communication about this, or how the service is now ran (it previously depended on Fastly's edge compute platform, which is not available on this chinese cloud).