I haven't seen much discussion of this on this forum but I've run into this issue several times. Since the requirement to field-mark certain electrical equipment to warn of potential arc flash hazards was first introduced in Section 110-16 of the 2002 National Electrical Code, and Arc flash markings were first mentioned in the 2004 edition of NFPA 70E, some people interpret this as equipment installed before 2002 must be labeled only if any modifications or upgrades are done to the equipment.

Has anyone run across any good articles which discuss this?

Does anyone use this as justification to not label equipment installed prior to 2002 that hasn't been modified?

The NEC is a forward looking document, and as such, does not require retroactive compliance.

Arc Flash labeling, to allow selection of proper PPE, is required by NFPA70E which does require 'retroactive' compliance.