I have a windows 2019 VM on a VXRail VSAN (vSphere 7) that has a 10TB disk with ReFS. This disk is used for only 1TB but it's now 11TB on vSAN due to ReFS not supporting TRIM/UNMAP. If I open the VM storage policy editor, even if I do not switch the storage policy, the web ui tells me:
In fact, it seems that it somehow understands that this disk could be reduced in size on the vSAN. There is a way to apply this change and have the vSAN fix the disk size?
I think you've answered your own question, "ReFS not supporting TRIM/UNMAP". vSAN doesn't know to do this unless the OS issues the TRIM/UNMAP command, or you Storage vMotion the VMDKs to different storage and then back again.
Now, when you change the policy from Thick to Thin, since it's clones the disks, it may be possible this will reclaim the storage, but I would have to put that to the test to be sure. Storage vMotion is the "tried and true" method to reduce the VMDK size.
ReFS does support TRIM/UNMAP when used with Storage Spaces, probably not an option in your case. To be honest, do you have a good use case for ReFS over NTFS? I find that ReFS has a lot of gotchas still and if not using it for Storage Space or files/volumes > 256TB, I would stick with NTFS honestly which fully supports TRIM/UNMAP and doesn't have all the gotchas.