The requirements state that all cluster systems must run the same OS. Does this include the same OS version level of a particular OS?
Running the same distro Linux OS on all systems but one could be version 12 vs 15 or simple different service pack level of the same major version. SP1 vs SP2 and so on. Would this be acceptable ?
Or with Windows, running 2016 and 2019 together in the same cluster.
probably (but non Splunk certified) a Splunk Cluster runs on little different versions (e.g. 14-15) of the same Linux.
I'd have many dubts about different Windows versions that are really different one by one (2016 is really different than 2019).
Anyway, I'd avoid to use Windows servers on production systems and upgrade of Linux systems to the same version isn't so heavy!
In addition I'd use the same os version also to avoid that at the first problem Splunk Support doesn't help you.
But this is only my opinion: Splunk says to use the same Operative system.