Hi Steve .,
The property should be available since Sitefinity 3.5 release. If you use Sitefinity 3.6 or 3.7 you should see this property for sure ( you can check our demo website at demo.sitefinity.com)
Categories Tree /TagsList controls are not IFilterable controls and datasource of these controls cannot be filtered out of the box. You will have to create a custom control in this cases. Here is a sample code for CategoriesTree
control that illustrates filtering by blog post.
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