We tried it, I even made some contributions trying to get it to work. In the end we just migrated to Mattermost, which speaks Slack webhooks natively and is a very competent Slack replacement. We self-host it, too, which is a big win.
https://zulip.com/integrations/doc/slack_incoming
It probably deserves another pass on the documentation; it's been improved a few times since then and may no longer deserve the "beta" designation.