In order to Poll a private channel, you will need to invite Polly to the channel (by using
/invite @polly). When you do that, Slack throws a message indicating that you’re adding a user, and this user will be able to read the history of the channel. The only channel we can read the history for is a 1:1 DM channel between a user and Polly. All other channels are safe.
While that in fact is true for a regular user (since they can scroll up and read those messages), Polly is a bot, and bot’s can’t do that, and don’t have programatic access to any history of the channel. While it is technically possible to read all future messages, we explicitly avoid that for your privacy by not subscribing to any events within the channel, except for messages related to @polly mentions, or polls that Polly has created.