When you edit a page, DokuWiki creates a revision with the old document. The old versions can be viewed by clicking the Old Revisions button. On the page shown, revisions can be compared with the diff tool.
Displaying the Old Revisions button can be disabled in the disableactions config option.
Revisions are stored within the attic directory, within the configured savedir.
The compression configuration option specifies if the pages will be saved as compressed files or not.
The default system path is <dokuwiki>/data/attic/<namespace>/<attic file>.
To remove any of the contents of the attic you can manually remove the corresponding files and sub-directories from the system.
To also remove the list from DokuWiki, edit the metafile
./data/meta/<namespace>.changes