What happens to the indexer cluster when the indexer Cluster Master (CM) runs out of disk space is that the indexer cluster will continue to operate as long as no indexers fail. The CM does not store any indexed data or participate in data replication or searching. It only manages the cluster by maintaining information about peer status, bucket status, and search head status. Therefore, if the CM runs out of disk space, it will not affect the normal functioning of the indexers or search heads, unless an indexer fails or needs to be added or removed from the cluster. In that case, the CM will not be able to perform its management tasks and may cause data unavailability or inconsistency in the cluster. References:

https://docs.splunk.com/Documentation/Splunk/9.1.1/Indexer/Systemrequirements#Cluster_master_node

https://docs.splunk.com/Documentation/Splunk/9.1.1/Indexer/Masterfunctions