You can make a post obsolete by following the below steps:
Click into the System Admin menu and select Post Admin.
Locate the post you wish to make obsolete and once you've found the post, click on it and a menu will open on the screen.
Click on the Details part of the menu and you will see a contract details.
Click Edit.
Tick the box for the Obsolete.
Click Save.
📌Note: This will remove the post from the organisation structure, however, the obsolete post will still show in the post setup if you need to refer back to it.