This quote answers some of the questions raised:
"Oversight was created as a stop-gap, to deal with the high-priority cases for high-level revision removal due to privacy issues. If revision deletion bitfields go into full use later, there won't be a compatibility problem; we'll just stop using the old oversight system." -- Brion, December 2006, http://www.mediawiki.org/wiki/Talk:Bitfields_for_rev_deleted