I know you've mentioned Slack multiple times, but please for the love of all things holy don't use a closed system that deletes messages and isn't easily discoverable via search engines. Zulip has built-in support for a providing SEO renders of the messages and I'd imagine one could easily self-host it in Oracle Cloud's free tier, or apply through their sponsorship program https://zulip.com/plans/#cloud-sponsorships

https://zulip.com/help/public-access-option and https://zulip.com/help/moderating-open-organizations are relevant