Reviewing purely from the perspective of how this document uses HTTP>

* Given that 'This document describes the issuance protocol for Privacy Pass built on [HTTP]', I suspect it should be a normative reference.

* 'The Issuer directory and Issuer resources SHOULD be available on the same domain.' Is "domain" a _hostname_, _origin_, or something else, e.g., using the Public Suffix List?
   
* 'Issuers SHOULD use HTTP caching to permit caching of this resource [RFC5861].' Either 'SHOULD use HTTP cache directives...' or 'SHOULD permit caching..'.
   
* Examples use HTTP/2; the style guide recommends using HTTP/1.1 for all examples except for those that are specific to a protocol version. See: <https://httpwg.org/admin/editors/style-guide>

* It's not necessary to specify Cache-Control on POST requests. 

* 'If any of these conditions is not met, the Issuer MUST return an HTTP 400 error to the client.' 

  - HTTP status codes should be spelled out; e.g., "400 (Bad Request)".

  - 422 (Unprocessable Content) might be a better status code to use here, though -- 400 will be used by generic HTTP software for problems at that layer, and so you won't be able to distinguish those problems from these more specific ones. 

  - Also, we generally encourage using SHOULD when specifying a status code like this, so that clients don't form the view that they can depend on seeing this status code in this situation (they can't; intermediary and other software may change the status code).
  
  - Have you considered defining one or more HTTP problem types (RFC9457) to provide more granularity and detail?