Comment 1 for bug 1791677

You can set the deletion_policy for a resource to 'retain'. (If you forgot to do that at the start, you can update the stack to include that in the template and as long as there are no other changes it won't affect anything.) For any resources with a deletion_policy of 'retain', the physical resource will be left in place but unmanaged by Heat when the resource (or indeed the entire stack) is deleted.

https://docs.openstack.org/heat/latest/template_guide/hot_spec.html#resources-section