Slack is a business communication platform.
On Slack's Privacy Page you can request and collect all the privacy and GDPR related information that Slack is providing.
The basis for Slack Technologies, LLC (a Salesforce company) sharing, adding, uploading, hosting, accessing or sending personal data out of Europe. Vendors can also add a description of their own impact assessment for data transfers.
Subprocessors are companies that provide a service for Slack Technologies, LLC (a Salesforce company) and who has or potentially will get access to personal data of their customers.