As I mentioned, in our community we have some deal with Slack so that we are paid (up to a point) and have full message history retention. Search can also go through all of those messages too. This is extremely valuable, and is a selling point for Slack if you can somehow get on to a paid plan without breaking the bank.

But in my experience, many don't use the search anyhow. :P

I'll admit, I don't know the full details of the relationship we have with Slack as I'm just an admin and not an owner. It is paid for by someone, but I don't know how much we're paying per head or of it's a flat rate.