Is there a rule that requires us to log that time, even as airline pilots that have already passed all their tests (ATPL, ETC.)?
As your profile states you're in the USA, presumably you're talking only about FAA regulation. If this is the case, the answer is no. No regulation requires you to log instrument time if you're not using it to meet the requirements of currency, a certificate, or rating.
Presently there is no requirement to log instrument time for currency. There used to be such a requirement, which was six hours of instrument time within the preceding six months, as well as six approaches, hold, tracking, etc. The requirement for logging the six hours of instrument time was eliminated several years ago.
You do still have a requirement to meet either your six month proficiency check, or six approaches (plus holding tracking, etc) in six months, and this must be logged. It must also be done under actual or simulated instrument conditions, and the approaches themselves must be flown in actual or simulated instrument conditions to minimums (per the FAA Chief Legal Counsel). These approaches must be logged as such in order to meet the requirements of the regulation.