Advertising metadata and other fields in overview

Hi all,
The LIST OVERVIEW.FMT command MUST return these 7 following case-insensitive strings:
        Subject:
        From:
        Date:
        Message-ID:
        References:
        :bytes
        :lines
However, RFC 3977 also states in Section 8.4.2 that "for compatibility with existing implementations, the last two lines MAY instead be:
        Bytes:
        Lines:
even though they refer to metadata, not headers."
Are you aware of news readers that send LIST OVERVIEW.FMT and would break if they encountered ":bytes" or ":lines"?  I'm wondering whether it is safe nowadays to advertise the expected strings.
Also, are there useful other header fields that news readers are already searching for in their (X)OVER requests and then would benefit from them if present in the overview?  The idea would then be to suggest to news admins adding such fields in their overview data.
Like Newsgroups to know whether the article is crossposted?  (Some news servers already provide the Xref header field, but one cannot take for sure that an article is crossposted with that mere field.)
--
Julien ÉLIE
« Tant qu'il y a des marmites, il y a de l'espoir ! » (Astérix)