Thanks :)
To dive a bit deeper, while XMPP/IRC exists, and it wouldn't be hard to make the Zulip server natively speak XMPP to clients, that in-and-of-itself wouldn't be quite useful. A message sent to stream "ops" with topic "prod deploy" might look like this†:
<message
from='coven@chat.shakespeare.lit/ops'
id='162BEBB1-F6DB-4D9A-9BD8-CFDCC801A0B2'
to='hecate@shakespeare.lit/broom'
type='groupchat'>
<subject>prod deploy</subject>
<body>Thrice the brinded cat hath mew'd.</body>
<delay xmlns='urn:xmpp:delay'
from='coven@chat.shakespeare.lit'
stamp='2002-10-13T23:58:37Z'/>
<addresses xmlns='http://jabber.org/protocol/address'>
<address type='ofrom' jid='crone1@shakespeare.lit/desktop'/>
</addresses>
</message>
But there aren't any existing clients that have the UI to:
1. Show per-message subjects/topics in a meaningful way
2. Allow users to easily reply to a message preserving the subject
So we'd need client changes to make it truly useful.
†: Adapted from syntax specified in XEP-0045
