Man page says
Timestamp files are created in the spool directory for each job in
anacrontab. These are never removed automatically by anacron, and
should be removed by hand if a job is no longer being scheduled.
However with a hundred of them it is accident prone to figure out which are
still being used or not visually.
Also if this needs to be done on a hundred machines it becomes tedious.
Therefore please at least add new functionality:
The option to --list-obsolete-timestamp-files is in release 2.3-44, which is relative to a spool directory and anacrontab file. Since other anacrontabs and spool directories can exist, these locations need to be stored before a delete option can be added.