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