closer-mop provides a way to do this for classes, which in practice are the most relevant types:
(c2mop:class-direct-subclasses (find-class'quri:uri))
As the name says, you get the direct subclasses. For the tree, you must apply it recursively.
I have this as a view on classes in my Web-based inspector: https://codeberg.org/khinsen/clog-moldable-inspector
