The free plan allows you only to keep the latest 10K messages. And it tends to be messy

And (IMO) it's sooo annoying that everything is hidden behind a login screen. I usually don't signup, just to see what might be behind.

Here're open source Slack alternatives for public communities, no login screens: https://spectrum.chat and https://www.talkyard.io (I'm developing it).

(Rocket Chat & Mattermost & Zulip are other alternatives, all require login to see what's there? )

Many organization and companies don't want outsiders to see what is happening due to privacy and security issues. Anyway, there is a PR opened for Zulip which allows people to see the content of public streams without logging in. It's up to the administrators to decide which streams would be public to users which are not logged in.

https://github.com/zulip/zulip/pull/8135

Interesting to know, thanks for mentioning :- ) B.t.w. I like Zulip's threads.

A thought: What if Zulip, during installation, let the admin choose if things should be public or hidden-behind-login, by default? E.g. useful for open source projects

We're actually working on something like this in Zulip; it's advertised as "coming soon" at http://zulipchat.com/for/open-source