The problem is that Slack and text messages don’t keep a public record. I could write a script that daily downloads Slack’s archive, converts it to HTML, and posts the HTML on a website. But I’d prefer a product, so citizens don’t have to trust me and my script.
Do you know of a service that does that? It doesn’t have to be Slack. Any instant messenger with good smartphone and web support would be acceptable. A way to record text messages (SMS or MMS) might also work.
Otherwise, I would think rocket.chat has this capability, albeit indirectly, where you can federate the data and/or allow public access to messages via API as well as to the audit record of the messages.