private boolean canTopicParticipateInReassignment(Uuid topicId) {
        return membersPerTopic.get(topicId).size() >= 2;
    }
If a topic has two or more potential members it is subject to reassignment.

@return true if the topic can participate in reassignment, false otherwise.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/assignor/UniformHeterogeneousAssignmentBuilder.java