Slack has enabled a new ‘Slack Connect’ feature that allows users to send messages or create shared channels with people outside of their organization. Slack has already disabled the optional message to be included in the invite to external users as it can be abused for phishing and abusive messages. The feature is not end-to-end encrypted, so other messaging apps such as Signal, Wire, or WhatsApp may be a better choice. Slack users can disable Slack Connect and prevent their users from accepting invites, users will still receive the invite emails.
Source: https://www.bleepingcomputer.com/news/software/slack-now-lets-you-dm-people-outside-your-company/