Discourse Policy plugin gives the ability to confirm users have seen or done something. Prior to version 0.1.1, if there was a policy posted to a public topic that was tied to a private group then the group members could be shown to non-group members. This issue has been patched in version 0.1.1. A workaround involves moving any policy topics with private groups to restricted categories.
https://github.com/discourse/discourse-policy/security/advisories/GHSA-jc5r-rm2j-mh4x
https://github.com/discourse/discourse-policy/commit/6b4390fe486408cc86ccea6b091406cfac6c5b8f