とりあえずこれが最新ルール…のはず
https://www.atptour.com/en/news/atp-announces-player-tournament-covid-19-support-package-march-2021

>Results from rescheduled 2020 events (Kitzbuhel, Hamburg, Rome & Roland Garros)
>will also be included for an additional 52 weeks at 50% from their originally scheduled drop date and be compared against the 2021 event in a ‘Best Of’ logic.
>The 2019 edition of these four events will drop from the rankings as per their currently scheduled drop date.