It’s not an issue with the client app, it has to be resolved on the Jabber server itself. The Jabber server is the one that connects to Yahoo! and passes the client information to Yahoo! DH would have to update the Jabber server Yahoo! transport to make it work… I can only assume it’s low on the priority list for them.
Last I read, Yahoo! keeps changing the way they authenticate users, or something. I think it’s been a while though since that has happened last though. Until then, you’ll have to use a separate client for Yahoo!