The length requirements only come into play when you are applying 210.12(A)(3) or (4).
210.12(A)(3) requires the circuit originate at a "listed supplemental arc protection circuit breaker". That breaker does not exist and probably never will as there is no standard to build such a breaker.
210.12(A)(4) requires that the circuit originate at an AFCI of the branch circuit/feeder type. That is the type of the original AFCI breakers and would likely cost just as much as the currently required combination type AFCI breaker.