When you delete a plan, you’ll be able to decide if you want to delete or keep the visits that you’ve created, to be able to use them again in a new plan.

When you delete a plan from the menu "Load older plans", you'll see this pop up:

You'll have to choose an option to continue.

If you choose to keep the visits, you'll be able to retrieve them by clicking on "From integration" on the Add visits menu (Step 2 of the route optimization module).