Add notice for end of support

Privacy Pass has changed since the original PoPETS version. Moreover,
CAPTCHA providers have moved from that version to follow IETF version
more closely. They don't operate with a stored token model, but with an
interactive token issuance and redemption, with a split attester/issuer
model.
This commit updates the README to highlight the end of active token
issuer/redeemer for this specific version of the protocol.

README.md

@@ -1,3 +1,9 @@
+# Deprecated
+
+#### This version of Privacy Pass extension is not currently under active development. More [details below](#deprecation).
+
+---
+
 [![github release](https://img.shields.io/github/release/privacypass/challenge-bypass-extension.svg)](https://github.com/privacypass/challenge-bypass-extension/releases/)
 [![Privacy Pass](https://github.com/privacypass/challenge-bypass-extension/actions/workflows/action.yml/badge.svg)](https://github.com/privacypass/challenge-bypass-extension/actions)
 [![License](https://img.shields.io/badge/License-BSD_3--Clause-blue.svg)](https://opensource.org/licenses/BSD-3-Clause)
@@ -102,6 +108,8 @@ npm test
 
 **2022** -- The Privacy Pass protocol can also use RSA blind signatures.
 
+<strong id="deprecation">2024</strong> -- The Privacy Pass protocol standardisation has diverged from the original PoPETS version, which this extension implements. To keep up with the protocol, CAPTCHA providers moved to this new version, and ended their support for PoPETS flavour. This repository remains as a relic of the past, but is not supported by any CAPTCHA providers. Cloudflare maintains [Silk - Privacy Pass client](https://github.com/cloudflare/pp-browser-extension) which forked this repository to provide IETF standard support.
+
 #### Acknowledgements
 
 The creation of the Privacy Pass protocol was a joint effort by the team made up of George Tankersley, Ian Goldberg, Nick Sullivan, Filippo Valsorda, and Alex Davidson.