Every chat app I've ever used going back to ICQ keeps permanent logs, except for Slack. (Technically Slack does too, it just keeps them on its own server and won't let you look unless you pay.)
Actually no: you can look all you like without paying, it's just kind of clumsy. If you "export your data from Slack" (yourteam.slack.com/services/export), you get the whole history, even on the free plan.
I've repeatedly considered writing a bot that would—on a regular schedule—poke this page with a headless browser to generate a dump, download said dump, and ingest it into ElasticSearch (which I'd then expose through a web search, or maybe just spit out batched archive pages into a static-site S3 bucket and let Google index them.) Such a bot would be a good companion to https://github.com/rauchg/slackin for FOSS teams.
But I haven't done any of that yet, because I get the feeling that putting enough attention on this little "feature" would get it quickly locked down.
