@vmb01, interesting question - I am not aware of any pertinent variance in the backend settings applied to node for each cluster type that would cause any difference here. 

If you have this already configured as stretched, you should be able to easily check this by creating the policy that would store data like this and validate whether it shows the vsanDatastore for this cluster as compatible (and if it does then validate by creating a test VM with this policy):

Nested Fault Domains are available for 2-node clusters, yes. But it requires at least 3 disk groups per host to have in-host RAID1:
https://core.vmware.com/blog/nested-fault-domain-2-node-cluster-deployments

Also interesting read: https://www.yellow-bricks.com/2022/03/17/stretched-cluster-witness-failure-resilience-in-vsan-7-0/

I have never seen a support statement around it to be honest,

Indeed we would not be able to find any docs about it.

You know, if the nested fault domain would be available also for 1+1+1 streteched cluster, we'll be able to compete with Symplivity and Datacore (I mean: from a data resiliency point of view)  

I don't see why it wouldn't be supported, even 2 node stretched is still 2 node, I would assume it is okay, but let me drop a note to a product manager and ask.

Hi Duncan, do you have any news, from the PM, about it ?

No news, just reminded him.