I believe this can currently be implemented manually via the event scripts. You'd have to manually invoke the appropriate curl commands from your host system via the event hooks that are currently provided, but it is possible.
Since functionality that's likely to be wanted with Slack integration would be very dependent on programmatic examination of the event, the users involved, and details of the review it occurred on, the complexity is almost as high as if the existing event hooks were used with manually written scripts.
As a simpler Slack integration I think the 3 major straightforward uses I'd like to see are (each optionally enabled):
1. Notification to a single Slack channel of all reviews (setup globally in admin)
2. Notification in private Slack channel between a bot and each author of a review for all notifying events in all reviews they're author on (configured via per-user toggle, does username mapping via remote integrations)
3. Notification in private Slack channel between bot and each participant of a review for all notifying events in all reviews they're a participant on (configured via per-user toggle, does username mapping via remote integrations)
Slack workspace API URL and token would need to be configured once at a global admin level.