This will be implemented soon, but doesn't have a high priority.
As stated in RFC-850/RFC-1036, the Followup-To header field is an optional field. Not required.
This is what the RFC says:
Certain headers are required, certain headers are optional. Any unrecognized headers are allowed, and will be passed through unchanged. The required headers are Relay-Version, Posting-Version, From, Date, Newsgroups, Subject, Message-ID, Path. The optional headers are Followup-To, Date-Received, Expires, Reply-To, Sender, References, Control, Distribution, Organization.
I doubt there are any news servers that reject messages without the optional Followup-To field.
In such case (if any) they don't follow the RFC.
Another quote from the RFC:
2.2.3 Followup-To
This line has the same format as Newsgroups. If present, follow-up articles are to be posted to the newsgroup(s) listed here. If this line is not present, followups are posted to the newsgroup(s) listed in the Newsgroups line...
So, again it's not a required field.