Limiting Workers to only be able to talk to servers that had "opted in" would be too limiting. Lots of people use Workers to talk to third-party APIs. And, many people are building whole applications entirely on Workers, with no origin server at all.

I'm guessing it's possible to amplify one request quite a lot which could annoy a victim server

We have other defenses against this. :)

`atob` and `btoa`

These are indeed awful and I wish browsers would define something better. :/