Please don't use Slack for open projects! It's a walled garden, search engines cannot index the content and registration is needed to even see the discussions.
What you put in there is basically closed from the public, complete opposit of openness.
What you put in there is basically closed from the public, complete opposit of openness.
Does matrix / someone else solve it? How?
Personally, i find Rocket.Chat ( https://rocket.chat/ ) to be simple enough to set up in a Docker container and have running as a passable alternative to Slack/Discord. There are downloadable clients as well. The only real downside I've seen is somewhat bad support for calls because their WebRTC implementation is still new and integrating with Big blue button or whatever else they offered is hard.
We provide our full Zulip Cloud Standard paid plan for free to open source projects (and various other worthy causes!). We also prioritize and build features specifically because OSS projects asked for them:
https://zulip.com/for/open-source/
Hundreds of open source projects, including Rust, Julia, MariaDB, and Clojure, use Zulip as their team chat platform. We often hear from large open source projects that they substantially prefer Zulip's topic-based model to the user experience in Discord or Slack, or that they found it made their open source community work. See for example these tweets from the last few weeks:
* https://twitter.com/mojavelinux/status/1409702273400201217
* https://twitter.com/ornicar/status/1412672302601457664
Also in contrast with open core products like Mattermost, Zulip is proudly 100% open source software. You can learn a bit more about Zulip's open source model here:
https://blog.zulip.com/2021/04/28/why-zulip-is-on-github-spo...
I'm not sure about this detail of Slack's ToS. At a technical level, you can certainly export your data from a Slack (which is we implement https://zulip.com/help/import-from-slack); I imagine it's easy to write a tool to format and publish it.
FWIW Zulip maintains https://github.com/zulip/zulip-archive, which is a configurable API-based tool for creating a static HTML archive from a Zulip organization, with tooling to update it every few minutes. A lot of larger open projects use it. (We're also working on a native logged-out access feature with less janky formatting, which has a working PR that we need to integrate).
I suppose you could export your data from Slack, import it into Zulip, and then publish that using zulip-archive if you didn't want to write any code, but I'm sure the formatting would be better preserved if one avoided the "convert Slack markup to Zulip markup" step.