ClassificationAncestor is a deprecated table, deprecated either after version 1.x (1.0 to 1.9) or version 2.x. Definitely not supported in version 3.x.

There are some triggers from the old days (but not necessarily all of them) that still update this table, but as new features were added, they don't update that table anymore as it is no longer supported.

Take a look at ClassificationHierarchy which is modern and may give you what you need.