The problem with "trying it with my team for one sprint" is that I now need to have a long conversation with legal. How will the service provider handle the content (discussions, screenshots, code snippets) that my team will upload? Do our current customer contracts contain any clauses that complicate the use of external discussion sites? In the unlikely event that the service offers the option to host it locally -- who is going to deploy it, configure it, set up security and authn/authz around it?
The real answer is: just fucking use email. Why is email not one of the options suggested in the TFA? It's universally available, durable, allows long-form conversation and thoughtful deliberation, doesn't tell you that "someone is typing," and offers you the luxury of checking it at a set interval only a few times per day.
The way many organizations have collectively abandoned email in favor of instant messaging is a travesty.
Slack and all its clones are based on the chat room model, which structurally has the problem described in this article (and many others for productivity, such as wasting attention). Fundamentally, the chatroom model pioneered by IRC is poor for asynchronous communication because you can't sustain temporally overlapping conversations in a channel.
However, you can't "just use email" -- Email's threading model is great for asynchronous work, but it is poor for synchronous communication and also doesn't support modern features that make it easier to communicate ideas efficiently (shared history, markdown formatting, image previews, emoji reactions, etc.). It's essential to be able to have (semi)synchronous written conversations with people you're working with, especially if you don't want to spend your days burning out on video calls.
This is why we created Zulip -- it's designed as a real-time communication tool with email's threading model and all the nice features of modern chat apps that email lacks. And the reading user experience is actually a lot better than email, because Zulip provides is designed with the goal of saving time when prioritizing, skimming, reading, and replying to conversations. It's also 100% open source software (not open core!).
We've been using self-hosted Zulip for about 18 months now, and it's exactly what our company needs.
I have been recommending it right and left to people who don't need federation in their chat system. If there was a clear gateway to federation -- enabling a Matrix room as a Zulip Stream, for instance -- I would enthusiastically recommend it for everyone.