Doktor Avalanche
From ezine:
 
Deleting a track will not remove associated Patch Points, so that undo and redo can
work. However, loading a project removes any unused patch points.
 
So will from Sonar menu - Edit -> History-> Clear (Undo history) remove unused patch points?

They're actually two separate things; the eZine statement relates to what happens to patch points when you delete tracks. Patch points by themselves are basically input and output assignments, which traditionally have not been included as events in the undo history. So like other input/output assignments, almost all patch point management is done manually. The "what happens if you delete a track with patch points" is more like a special case that's accommodated. 
 
For example, suppose you add a patch point input to a track (we'll call it Track X). You assign several outputs to it. Then you de-assign those outputs and save the project. You might think that because nothing is assigned to Track X's patch point, the patch point will be deleted but that's not the case. If you re-open the project, that patch point will remain because you might want to assign something to it.
 
Now suppose you retain an output assigned to Track X, but de-assign Track X's patch point input. Track X now becomes an audio track because it no longer has a patch point input. If you save the project and re-open it, because the patch point still exists as an output (even though it's not assigned to anything), it will still exist and if you want, you can re-assign the Track X input to it.
 
However, if you de-assign the patch point input to Track X, and de-assign the output going to it, then save the project, when you re-open the project the patch point will no longer be available because nothing refers to it. If you want to re-patch the output to Track X, you would need to create a new patch point. So with a few exceptions, you can basically think of patch points as being very similar to input and output assignments.