Zulip's hosted plans for corporate users have a similar model to Slack's -- there's a free plan with limited access to history, and a paid plan with full access to everything.
Since you mentioned nonprofits, we provide free hosting for open source projects and free or highly discounted hosting (e.g. 15% price) for many other nonprofits.
So Zulip doesn't hide chats over a certain amount like Slack does eh? This could make it extremely useful for cash strapped non-profits.