I contacted support to opt out.. Here is the answer.

"Hi there,

Thank you for reaching out to Slack support. Your opt-out request has been completed.

For clarity, Slack has platform-level machine learning models for things like channel and emoji recommendations and search results. We do not build or train these models in such a way that they could learn, memorize, or be able to reproduce some part of customer data. Our published policies cover those here (https://slack.com/trust/data-management/privacy-principles), and as shared above your opt out request has been processed.

Slack AI is a separately purchased add-on that uses Large Language Models (LLMs) but does not train those LLMs on customer data. Slack AI uses LLMs hosted directly within Slack’s AWS infrastructure, so that customer data remains in-house and is not shared with any LLM provider. This ensures that Customer Data stays in that organization’s control and exclusively for that organization’s use. You can read more about how we’ve built Slack AI to be secure and private here: https://slack.engineering/how-we-built-slack-ai-to-be-secure....

Kind regards, Best regards,"