Mitigating the Hetzner/Linode XMPP.ru MitM interception incident

They point out that "CT is optional", but isn't that ultimately a decision by the XMPP clients? What stops them from requiring CT now that browsers effectively do? What trust store would they be relying on that issues certificates that don't work in the browser?

Right, they could do this and this seems like a good idea to me. The problem this require-CT behaviour isn't switched on by default for basically any TLS library, so an XMPP client developer would have to go out of their way to switch this on, and I assume most or all (currently) don't. That can change of course.

How problematic is that in real life? All CAs worth their salt are already publishing all certificates to CT logs because most TLS servers are browsers and most clients are browsers.

Maybe there's a CA somewhere that has opt-in CT logging enabled, but I don't think they're very popular. For example, even the rogue CA certificates were published onto the CT log (because Let's Encrypt does so by default).

I think a CT check shouldn't be a very problematic change in practice, as long as there are reasonable exemptions (i.e. don't require CT for certificates imported into the CA store, allow the user to disable the check, etc.). For server<->server communication things become more difficult, but the entire network would be better off if a major server like ejabberd were to require CT.