Privacy Pass. https://privacypass.github.io/

How does Privacy Pass work exactly, to not enable spammers and other malicious bots?

There's a pretty good write up on how it works here: https://blog.cloudflare.com/cloudflare-supports-privacy-pass...

This is great, why isn't this part of the default Tor Browser Bundle?

As I understand it, it is part of the default Tor Browser Bundle. The problem is for people using other browsers with Tor.

I couldn't see it in the list of extensions there. I saw a ticket filed that said that there are various arguments against including it, one of them being that it's not the responsibility of the user to install some extension in order to get unblocked, it's the responsibility of the provider to not block users.