I'm not sure why people claim IRC lacks archiving support. Isn't a bit like saying SMTP lacks archiving support? And doesn't basic IRC always go through a server? So there shouldn't be any technical barrier against a server archiving all chats (private and in channels)?

As for NNTP, I'm not sure if NNTP over TLS, peering with only trusted sites (aka for internal use) would make sense or not. I never did use Usenet much. At least the D language forums have made an effort to bring NNTP into the www era[d].

It's interesting that no one seems to do a decent job of (server side) archiving for XMPP -- partly I think it's because as you state, the XEPs have gotten out of hand -- and partly XMPP appears to be especially popular for users that want privacy -- and treat ephemeral chat as a feature.

[d] https://github.com/CyberShadow/DFeed

(format note, you probably should've just listed the protocols in separate paragraphs, as indented blocks with lines longer than ~50 characters doesn't format very well on hn).