The ack3 GitHub project is at https://github.com/beyondgrep/ack3.
Bugs, feature requests and other issues can be submitted to https://github.com/beyondgrep/ack3/issues.
beyondgrep.com has a dedicated Slack at https://beyondgrep.slack.com/. There you'll find an #ack channel.
The channel is currently invitation-only, so email firstname.lastname@example.org to get an invitation sent to you. Soon we'll have it set up to the general public.
There are three mailing lists for keeping up on the world of ack.
Main list for discussion of ack for both ack's users and those looking to help contribute. New fans of ack may want to browse the archive for cool tricks.
Very low volume list for announcements of new ack releases.
List for discussion of Ack's development - intended primarily for its developers. This is in addition to the ack2 GitHub project.