Maybe we can remove this flag / feature with PMD 7. It tries to rewrite rule references to rules, that have been moved and don't exist anymore. It's long time not updated anymore, and we mostly use https://pmd.github.io/latest/pmd_devdocs_rule_deprecation_policy.html now (providing deprecated rule references).
In PMD 7, we have a lot better error reporting mechanism (thanks to @oowekyala) and when we tackle to change the ruleset format, we will anyway need a whole different solution as a compatibility layer...