In the details of the abuse filter log -- such as http://test.wikipedia.org/w/index.php?title=Special:AbuseLog&details=1784 -- one can clearly see the presence of the old_text and old_html variables. These variables are clearly documented at http://www.mediawiki.org/wiki/Extension:AbuseFilter/RulesFormat
However attempting to use these variables in a filter results in a syntax error for unknown variables:
"abc" in old_text
Syntax error detected: Unrecognised variable old_text at character 8
new_text and new_html work fine.
This is a problem because to use new_text or new_html, a proper filter often needs to also check old_text or old_html, respectively. Enwiki in particular is dealing with a particular vandal against whom new_text is the only appropriate choice, but without old_text this filter is encountering false positives.
Looking at the source code, old_text and old_html already appear to be implemented, it just seems that old_text and old_html are just not recognized by the abuse filter parser for some reason.