Then over the last few years, a slew of new chat based apps popped up in the enterprise that were primarily focused on group/channel chat (e.g. Slack, Zulip, etc.) as opposed to the 1:1 messaging model.
What would be good chat software today that focuses on 1:1 messages (not groups/channels)?
[XMPP worked great for this but that doesn't seem en vogue any more. And I find the focus that many of these "modern" chat app have on group/channel distracting]
The premise of the question is that direct messages are somehow different and require a separate application. I don't think that's true. Slack and Teams do an absolutely fine job at 1:1 messages.
I don't think even XMPP focused on 1:1 and even Slack uses XMPP (or at least used XMPP in the earlier versions, as it started out as an internal communications solution built as IRC + storing messages in database + search + ...)
Also, https://meet.jit.si , https://jitsi.org They also use XMPP.
Preferably only Telegram, but I don't have a choice.
or Jitsi.