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.
Controversial take: We found that we get better responsiveness from our open source community on Slack at PostHog, probably as our users have it open for work already and are used to posting without too much thought, whereas making a post on a forum feels like a higher barrier to entry. This feedback helps us improve the product. Would love something that extracts all the content and pushes it somewhere indexable though, as we often end up manually taking conversations and converting into github issues etc.
Zulip, which I lead, absolutely aims to solve this problem.
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:
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:
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:
Gitter seems to be indexable by search engines. My experience using it as part of open projects has been mixed—the UI isn't great and the notification system is, in my experience, unreliable—but if you're worried about your messages being discoverable, it might be up your alley.
I'm also curious about this. Gitter.im (which now is integrated with Matrix) is also an option, but not as user-friendly as Discord. With Discord at least you have unlimited searchable chat history (which is invaluable for OSS communities, and one of the top reasons you should avoid Slack). However, I can see Discord removing this free luxury in the future...
Well, there are plenty of open solutions that you can self host.
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.
Does Slack's terms of service allow for a plugin that can just export the contents of the Slack to a public site (like a mailing list archive)? Do Slack plugins have that kind of access?
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.
What you put in there is basically closed from the public, complete opposit of openness.