It doesn't. Big companies who want to be gdpr compliant buy enterprise Slack and force slack to store the data in European datacenters. Slack doesn't give a shit about gdpr if you're not paying
It probably does. But it requires someone to pick the battle (a customer or an enforcement agency on their own) to get it enforced.
Depends: Do they not delete the data if you explicitly contact them and tell them to do it, or do they just not give you a convenient button in the UI?