To make an absence class obsolete, you need to follow the below steps:
Click Settings, usually under the Configuration tab then click Module settings.
Click HR then click into Absence class.
This will show a list of all absence classes
Click into the desired absence class
Select the Obsolete box, then the absence class will no longer be available for selection.
📌Note: If you unselect it, then the absence class will be available again for selection.