according to official documentation, all indexer cluster nodes (master node, peer nodes, and search heads) must run on the same operating system (official documentation). It's not clear to me if this mean the same operating system type (e.g. SLES) or the same operating system type in the same version (e.g. SLES v12.3).
We are going to "upgrade" the cluster master and some search heads to a newer operating system version, creating new machines that will substitute the existing ones. We would like to move from SLES v11.4 to SLES v15.1, but indexers are running on SLES v12.3.
Mixing SLES v.12.3 (on indexers) and SLES v.15.1 (on cluster master and search head) could cause issues? In any case, we would install the same Splunk version on all cluster nodes.
as long you are using enough close versions that should work. This enables you to upgrade OS to the newer versions. But to avoid additional weird issues, I suggest that you could upgrade those to the same major, minor and patch levels as soon as possible.