>> But the old behavior is undesirable, why continue it? > > Purely for backwards compatibility.
As discussed off list, the value of the obsolete variable org-preview-latex-image-directory now overrides the :image-dir specified in org-html-latex-image-options, but only if the former has been modified from its default value. This is implemented in the previously added option filter org-html-latex-override-image-options in the same commit that this filter was introduced, and force-pushed to the working repo.
