For every teams that are using Slack. Be sure that you are ok with the Privacy Policy of the platform.

We may employ third party companies or individuals to process personal information on our behalf based on our instructions and in compliance with this Privacy Policy. For example, we may share data with a security consultant to help us get better at preventing unauthorized access or with an email vendor to send messages on our behalf. We may also share data with hosting providers, payment processors, marketing vendors, and other consultants who work on our behalf.

We may also share aggregated or de-identified information with our partners or others for business or research purposes. For example, we may tell a prospective Slack customer the average number of messages sent within a Slack team in a day or may partner with research firm or academics to explore interesting questions about workplace communications. Again, this policy is not intended to prohibit the disclosure and use of aggregated or de-identified data.

All the communication made with Slack are saved on their servers in a way that they can have access to everything in a quite simple way (including conversation history, files…).

And to clarify the last sentence in bold, if I understand correctly (correct me if I'm wrong) : "this policy allow the disclosure and use of aggregated or identified data (with our partners)".