Judging from the IETF mailing list it was forgotten inside the RFC document.

Relevant mail is the second in the thread:
>Good point.
>
>DTLS is intended to support extensions--and OpenSSL, at least,
supports them in the same way as it does for TLS.
>
There probably should be a definition of ExtendedClientHello
in 4346 and 6347, but it's exactly the PDU you would expect.
I.e., the extensions come after the CompressionMethod.
>
-Ekr



Thread search results [here](https://mailarchive.ietf.org/arch/search/?q=%22DTLS+lacking+TLS+extensions%22). Contents here:  

[[TLS] DTLS lacking TLS extensions ?](http://www.ietf.org/mail-archive/web/tls/current/msg08404.html), Martin Rex (Archived [here](https://archive.today/W35sp).)   
  [Re: [TLS] DTLS lacking TLS extensions ?](http://www.ietf.org/mail-archive/web/tls/current/msg08405.html), Eric Rescorla (Archived [here](https://archive.today/5kqGw).)  
    [Re: [TLS] DTLS lacking TLS extensions ?](http://www.ietf.org/mail-archive/web/tls/current/msg08406.html), Martin Rex (Archived [here](https://archive.today/1gQv8).)  
      [Re: [TLS] DTLS lacking TLS extensions ?](http://www.ietf.org/mail-archive/web/tls/current/msg08407.html), Marsh Ray (Archived [here](https://archive.today/x5J7X).)