Original text:
3.  This document defines a new extension type (compress_certificate(TBD)), which can be used to signal the supported compression formats for the Certificate message to the peer.  Whenever it is sent by the client as a ClientHello message <del> extension (RFC5246, Section 7.4.1.4), it indicates the support for compressed server certificates.  Whenever it is sent by the server as a CertificateRequest extension (I-D.ietf-tls-tls13, Section 4.3.2), it indicates the support for compressed client certificates.  Note that CertificateRequest messages are not supported by TLS versions prior to 1.3. </del>  By sending a compress_certificate extension, the sender indicates to the peer the certificate compression algorithms it is willing to use and try to validate the Certificate message exchanged during a TLS handshake.  This means that middleboxes that don't understand the CompressedCertificate message might misbehave and drop connections <del> that adopt certificate compression. However this is not a problem when using TLS version 1.3 I-D.ietf- tls-tls13 and higher, due to the fact that the Certificate (and thus the CompressedCertificate) message is encrypted, preventing middleboxes from intercepting it. </del>  7.

Reviewer comments:
We already had the understanding that no one will attempt to deploy the extension for TLS 1.2 and earlier, since it would be plagued with middlebox compatibility issues that TLS 1.3 already faces. However, it was left as supported because there was no cost to supporting it. On the list, David Benjamin has pointed out that: clients have no way to ask for "compress only in TLS 1.3" policy, meaning that they have to assume that a TLS 1.2 connection could attempt to use this extension; clients have to support certificate compression in TLS 1.2 as a result. This adds complexity for no reason, and the problem is easily solved by restricting the extension to 1.3 or higher.
:+1: LGTM

Revised text:
3. <ins> This extension is only supported with TLS 1.3 and newer; if TLS 1.2 or earlier is negotiated, the peers MUST ignore this extension. </ins> This document defines a new extension type (compress_certificate(TBD)), which can be used to signal the supported compression formats for the Certificate message to the peer.  Whenever it is sent by the client as a ClientHello message  <ins> extension (I-D.ietf-tls-tls13, Section 4.1.2), it indicates the support for compressed server certificates.  Whenever it is sent by the server as a CertificateRequest extension (I-D.ietf-tls-tls13, Section 4.3.2), it indicates the support for compressed client certificates. </ins> By sending a compress_certificate extension, the sender indicates to the peer the certificate compression algorithms it is willing to use and try to validate the Certificate message exchanged during a TLS handshake.  This means that middleboxes that don't understand the CompressedCertificate message might misbehave and drop connections  <ins> that adopt certificate compression.  Because of that, the extension is only supported in the versions of TLS where the certificate message is encrypted in a way that prevents middleboxes from intercepting it, that is, TLS version 1.3 I-D.ietf-tls-tls13 and higher. </ins> 7.