The ALC42s AFAIK still are not approved for ACSES. As a result they cannot lead on the Lake Shore Limited or some of these northeast state corridor routes like the Valley Flyers. The Lake Shore could very plausibly be the last long distance route that sees them unless the approval is right around the corner. It isn’t quite clear yet to what extent the diesel ALCs will be used on routes that will eventually see the hybrid variant and Airo trainsets. Some others may see them as they are running on the Palmetto and have made appearances on the Carolinian as well which are both slated to get the trainsets. They may try to have them make their way around to most places in the interests of getting mechanical staff and operating crews trained on them since the hybrid locomotives in the Airo sets will be very similar. One corridor route that will almost certainly also eventually get them is the Heartland Flyer which is being considered to be covered by the long distance order for re-equipping as opposed to the Airos.I know the Pennsylvanian, Cardinal, Crescent, LSL, as well as Valley Flyers are still using P42s. Some Northeast Regional trains use them as well. These are just a few routes off of the top of my head that I can think of that still use P42s.
I know the Cardinal and Crescent still have them, but sometimes mixed in with chargers.