The Slack integration is implemented as a bot, which means it requests several permissions. Before you connect the integration, Slack will present a list showing the exact permissions:
If you expand the Perform actions in channels & conversations section, you'll see that the bot has the ability to message anyone in the workspace:
Although this permission is requested, the bot will only send direct messages to people who have installed it individually.
The bot will also post a message to a public channel if someone has decided to share a Kudos to that channel.
The bot will not read any messages in your Slack instance other than messages sent to it in a DM channel.
Currently, our Slack integration is implemented as a classic bot, meaning it requests a broad set of permissions, most of which are not used. By October 15, 2020, we plan to migrate our integration to the new granular permission system so that it requests only the permissions it needs. For details on the new permission system see the article here: https://medium.com/slack-developer-blog/more-precision-less-restrictions-a3550006f9c3