In the last decade, decentralized web-crawlable forums (the likes of vBulletin, phpBB, etc) have been decimated by the proliferation of monolithic walled gardens like Discord and Slack. Then Zulip comes along with features that feel like a throwback to the forum era (web frontend, public access option, threaded topic system, email notifications) but with modern sensibilities in mind (mobile clients, proper API, bots, push notifications). Zulip seems like the kind of project that has the potential to turn the tide and make discourse more decentralized again.
I fear that the introduction of any pricing for self-hosted installations like this will steer Zulip away from its potential as a ubiquitously deployed platform and toward a niche subscription model, as open as it may be, relegated to the odd enterprise with refined taste in communication platforms. Very unfortunate news.