Messages and files, for platform-level machine learning.
Only an owner can do this, so check before you spend time on it.
A workspace owner or primary owner. An individual member cannot opt themselves out, and telling them otherwise wastes their time. Detect this and say it up front.
Messages, files, and usage data feed platform-level machine learning such as search ranking and recommendations. Slack states that customer content does not train generative models sold to other customers.
Historically this has been an email from a workspace owner to Slack's feedback address naming the workspace URL and requesting exclusion, rather than a setting in the admin panel. Verify the current mechanism before shipping this entry as actionable.
Mark it blocked, not skipped. Blocked means someone else has to act, and it should show up differently in the summary.
The old help article 404s. Workspace owners only, and historically by email rather than a setting. Confirm the current mechanism before treating this as actionable.
Every service on that list buries its opt-out somewhere different. Don't Train On Me puts all of them in one checklist in a panel beside your browser, walks you through them one at a time, and helps you confirm your privacy settings on the ones that expose a setting it can read. Free, no account, and nothing you tick leaves your machine.