I'm not sure about this detail of Slack's ToS. At a technical level, you can certainly export your data from a Slack (which is we implement https://zulip.com/help/import-from-slack); I imagine it's easy to write a tool to format and publish it.

FWIW Zulip maintains https://github.com/zulip/zulip-archive, which is a configurable API-based tool for creating a static HTML archive from a Zulip organization, with tooling to update it every few minutes. A lot of larger open projects use it. (We're also working on a native logged-out access feature with less janky formatting, which has a working PR that we need to integrate).

I suppose you could export your data from Slack, import it into Zulip, and then publish that using zulip-archive if you didn't want to write any code, but I'm sure the formatting would be better preserved if one avoided the "convert Slack markup to Zulip markup" step.