Hello, I'm trying to get rid of some old sketch, but If I delete it, all feature will be deleted as well, even when the sketch is no more in use in the feature tree.
Any idea why?
It means there is still relation to old sketch in your latest features. For ex. you have created sweep1 feature using plane 1 as reference. But if plane 1 has reference to Sketch3, then you have a reference trail. So if you try to delete Sketch3, you will have a domino effect. Try to move Sketch3 down the model tree, when it can't move it below a specific feature. That's the feature which has reference to sketch 3.
Tools - Relations - Display Delete
If you use this tool while in a sketch, you'll see all of the relations in it. Clicking each relation will highlight the elements it uses.
It is not a perfect system for figuring out how/why a sketch has relations to another item though. You the user are the best tool to prevent these accidental relationships. Be very thoughtful when adding relationships in a parametric modeler. They, along with your dimensioning scheme dictate how the model behaves in the future.
I think it is a good idea to delete unused sketches, but in this case your sketches are being used. Another option is to simply hide them.
Here is the screenshot for better understanding
It seems that the problem is due to a reference point included in the sketch3 used into the plane1.
But how do you find out "point2@sketch3" inside sketch 3? There is no information on sketch 3 regarding this point!