Your biggest practical concern is trying to migrate VMs. But, if you have a cluster with mixed OS nodes, all bets are off.

In practice, they don't usually look that closely and things just work. On paper, they support the same level and downward to the oldest Microsoft OS until it's end-of-life. On paper, Microsoft supports any Windows or Windows Server operating system inside a virtual machine as long as it is no further than one version past its host. The operating system inside virtual machines does not matter. And by 'anything', that could even include things that have nothing to do with the operating system levels. Mixing them will probably work, but if anything goes wrong, you're probably going to have trouble getting help. This mode is specifically meant to allow you to upgrade or replace the members one at a time until the cluster is homogenous, then update the functional level.

In no case are mixed-level clusters supported long-term. So, if you've got 2012 R2, then nothing after 2016 is supported in the same cluster. The maximum allowed distance between host operating systems is 1. Mixing node operating systems in a cluster is only supported for a rolling cluster upgrade scenario.

Windows 12 r2