Users can see all groups in the cluster (using consumer group’s -
list option) provided that they have Describe access to the cluster. It would make more sense to modify that experience and limit what is listed in the output to only those groups they have Describe access to. The reason is, almost everything else is accessible by a user only if the access is specifically granted (through ACL -add); and this scenario should not be an exception. The potential change would be updating the minimum required permission of ListGroup from Describe (Cluster) to Describe (Group).
We can also look at this issue from a different angle: A user with Read access to a group can describe the group, but the same user would not see anything when listing groups (assuming there is no Describe access to the cluster). It makes more sense for this user to be able to list all groups s/he can already describe.
It would be great to know if any user is relying on the existing behavior (listing all consumer groups using a Describe (Cluster) ACL).