I held the Killer (old) speedrun record on Bubba and then it got legacied, but now I see I'm 4th on new, and there's a run from 4 months ago on old? What happened?

That is because the 'Killer (Old)' category was originally used for all speedruns submitted before patch 9.0.0, following the changes made to hooks (which were later restored to the main 'Killer' category since they were improved in a subsequent patch). Additionally, we have included—and will continue to include—runs featuring add-ons and perks that were nerfed or reworked, as well as runs that slightly break current rules. This is why you might see a 5-month-old Leatherface run in 'Killer (Old)', while some runs from 5 years ago or more remain in the main 'Killer' category.