{
Schema: map[string]*pluginsdk.Schema{
...
    "prop_set": {
	    Type:     pluginsdk.TypeSet,
	    Elem: &pluginsdk.Resource{
		    Schema: map[string]*pluginsdk.Schema{
			    ...
    
			    "prop_list": {
				    Type:     pluginsdk.TypeList,
				    Elem: &pluginsdk.Schema{
					    Type: pluginsdk.TypeString,
				    },
			    },
		    },
	    },
    },
...
},

...

CustomizeDiff: pluginsdk.CustomizeDiffShim(func(ctx context.Context, diff *pluginsdk.ResourceDiff, v interface{}) error {
...
    value, _ := diff.GetOk("prop_set")
...
}

...
}