I completely agree with you on slack for open-source projects. Especially with Mattermost being so good, there’s no good reason for a project to stick to Slack.
Even Mattermost doesn't offer free accounts to open source projects[1], and I think it's the _hosting_ that keeps projects off of almost any of the open source solutions. But one of the principals of Zulip almost always weighs in on discussions to point out that they have free hosting for any open source project that wants to use it: https://zulipchat.com/for/open-source/#free-hosting-at-zulip... and seem to even offer "import from slack" to make it less painful to switch.
