However, OS and non-profit communities quickly run into the history issue. There is no way that some of the communities I'm part of on Slack could pay for all the users. For example, the "Maker's Kitchen" slack has 1500-2000 people.
How much information and knowledge is lost and questions re-asked?
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.
This is common, and Slack is treated more like a hallway conversation. Hallway conversations are enormously valuable, but if you want a record of something or if you want to make sure it reaches everyone, you use email instead.
I even went a step further and built a product that would generate FAQs from slack history and made it easy for people to add and categorize knowledge from Slack to provide answers to questions, metric, etc...
I would love to have open source communities use it for free if helped them manage supporting their communities easier.
It's https://landria.io. Would love to know if it would be useful for you.