I bikepack on a Moots YBBeat. It’s a soft tail that has 1.5” of travel on the rear. It’s just enough to take the edge off hits but doesn’t have any pivots to worry about. My packing varies from trip to trip but the bike is the same.
Here it is in all its naked glory. I had rack mounts added to the bike as they usually didn’t come with them.
There are several variations of this kind of rear suspension from the mid to late 90s. Mine is a 1998 but there are other soft tails to look at. Moots still makes the
Mountaineer YBB but they are quite expensive for just the frame…north of $8000
Another dually I’d consider is a Specialized Epic. I haven’t used one for bikepacking yet but I have one in Arizona that I’m going to try in February. It has the Brain rear suspension that is rigid while you pedal and opens up when it hits something. Very nice for climbing and the rear suspension isn’t always “inch worming” like more active suspensions.
This bike is missing the rear rack, of course but there now several rack attachments that could be used.