What exactly "flies with soul" are, I don't know, but much like pornography, what I know them when I see it/them. It is not my term but something Kelly Galloup had talked about in a video. When you say there are flies that have no soul, you know you are going to get a lot of push back. From my viewing of the video, he is not calling soulless flies bad - he fishes them - but rather he has "a thing" for soulful flies, as do I. I do not know that being old necessarily gives a fly soul - but it generally helps, I suppose.
I am 100% with Kelly here - flies like Peridgons have no soul. I don't know that Kelly has a definition of what creates a fly with soul and what characteristics preclude a fly from possessing soul. Though it seems beads are one of the criteria that prevent a fly from having soul - a designation I don't fully agree with. And mylar is particularly singled out - and I get it. To me, soulless flies lack movement. They are hard and impersonal. What exactly makes a fly "personal", I am not sure I can adequately describe - but I know personal when I see or fail to see it.
I don't know that this is Kelly's criteria but for me, a fly with soul has some "flow" to it and a bit of softness - or at least a lack of being overly rigid. He singled out mylar and beads - but only some flies with beads. His first example of a soulless fly was a Perdigon - which I totally understand. Whether you fish and like them or not, they lack soul. They were created and gained popularity because they are fish catching machines - not because of their soul. They are - in my mind - the ultimate soulless fly.
In the fly fishing world, it is not just flies we talk about having or lacking soul. Many fly rods lack soul whereas bamboo and fiberglass rods have soul. But not all graphite rods lack soul. I have an old Sage Lite Line (LL) that has plenty of soul. The Sage RPL that I built with my great uncle George Close nearly 30 years ago now is a rod I love, but it lacks soul - and I am OK with that. For a fly rod, soul, in my opinion, is about the connection you have with it. A rod that you can feel flex on every cast and every fish you catch has soul. As a rather "aggressive" caster, I am only fairly recently learning to better appreciate.
Without much question, my favorite fly is Hans Weilenmann's CDC and Elk - I am pretty much the Johnny Appleseed of the CDC and Elk in Wisconsin. While soul is subjective, it is unquestionably a fly that has abundant soul. What I love about the fly is its versatility and its movement in the water. The soft and wispy CDC fibers move like little else in the water. Hell, adding a CDC collar to a Perdigon at least makes one talk about whether or not that fly has soul (it doesn't...).
I think we might be too quick to think that dry flies have soul and that it is much more unlikely that streamers and nymphs have soul. But I do not think it is that simple. While I love the Morrish Hopper, Chernobyl flies, the soullessly named Hippie Stomper, and other foam flies, they lack soul. but a simple foam beetle with a peacock underbody is at least in the conversation of having soul. Thinking about mayfly imitations, Lawson's No Hackle has more soul than a Sparkle Dun, my favorite mayfly imitation. A Catskill mayfly with split upright wings has more soul than a thorax fly or a parachute fly.
I am starting to feel like an arbitrator of soul, which is not my intention. In all honesty, I do not much care. Fish flies with or without soul - I do not care. Disagree with my views on what flies have - or lack - soul; I don't care. In fact, I fish plenty of flies without soul. And I can't say I enjoy fishing soulless flies any less. But at the same time, I do have a bit of appreciation when I am fishing a more soulful fly on a soulful rod. It is only fairly recently that I have fished fiberglass more often. Fiberglass rods (generally) have soul. You can feel the fish into the handle.
Fish the flies, the rods, the places that make you happy. Count fish or don't count fish. Fishing should be about having fun and doing what makes you happy and make you feel fulfilled. Like me, you may prefer to limit yourself to dry flies as much as you can and work to figure out how to trick fish in this narrow place. Or chase that "PB" fish with big-ass streamers. Euro-nymph with soulless flies. I do not care - just do what makes you happy and satisfied.