In the current setup where a Slack-channel is mapped to a Zulip-topic it's not possible to distinguish Slack-threads.
What would be a solution to still follow threads?
When a Slack-message is part of a thread it could be posted to Zulip with a search-link included, e.g.
[thread abcd](https://clojurians.zulipchat.com/#narrow/stream/182792-slack-archive/topic/crux/search/.5Bthread.20abcd.5D) original message. Clicking this link would yield the thread via the Zulip-search.
The first message of the thread should be edited as threads are created retroactively.