Do not unfurl the slack notifications to unauthorized members of slack channels.

Summary

Please, change the general Slack message to Slack channels for issue notifications when there are issue security levels associated with the issues. The unfurled Slack message contains some details of the issue, like the issue summary, project, etc, that shouldn't be visible to users who are not at the issue security level.

Diving into the Slack API documentation, Slack has a feature to send messages to individual users in a public channel (they call it postEphemeral). The Slack integration would need to replace the current post message to channel with multiple postEphemeral messages to a given list of users.

Workaround
Create individual channels for members of each issue security and configure slack notifications to those channels filtering by issue security level.