I've found this quote in what appears to be the Usenet
sci.lang FAQ page:
Earlier historical linguists cheerfully reconstructed eight cases for PIE, on the model of Sanskrit; but the IE languages with many cases are now considered to be innovative, not conservative. The other cases developed from postpositions or derivational suffixes. Luwian, a sister language of Hittite, for instance, has no genitive, but has an adjective-forming suffix -assi, as in harmah-assi-s 'of the head'. (This is an adjective, not a genitive, because it can be declined.) Genitives in other languages often seem to be developments of cognates to this suffix.
(Emphasis from original). "Other cases" here seems to refer to anything other than subject and object.
Most other sources I find do list 8 cases for PIE. Is the number of cases in PIE subject to disagreement today? If not, was the view that it had fewer cases ever mainstream?