Sometime between December 4th…
https://web.archive.org/web/20241204202205/https://corsproxy.io/
…and December 5th…
https://web.archive.org/web/20241205230048/https://corsproxy.io/
corsproxy.io started charging for access and returning 403s. (Based on
the claim of 900TB/mo of traffic, that's probably not terribly
surprising!)
For now, we switch to another free one. At some point, I'll probably
set one up of my own to not have to deal with this again, but I'll need
to figure out how to deal with authentication first to prevent abuse.