They offer a thing they’re calling an “opt-out.”

The opt-out (a) is only available to companies who are slack customers, not end users, and (b) doesn’t actually opt-out.

When a company account holder tries to opt-out, Slack says their data will still be used to train LLMs, but the results won’t be shared with other companies.

LOL no. That’s not an opt-out. The way to opt-out is to stop using Slack.

https://slack.com/intl/en-gb/trust/data-management/privacy-principles