During the late 1950s and 1960s, I used Fenwick FF85 rods. There was no question that after long and hard use, the blanks would soften and the action would slow a bit. I had Temple Fork Outfitters check with legendary rod builder Gary Loomis, who replied that modern fiberglass rods will not soften or change in action simply from being cast. Nice to know.
I suspect (& hope) that he does.
In years past, most fiberglass rods were made with woven fiberglass and polyester resins. The weave had a natural sleaze, which combined with lower strength polyester resins would allow the composite to very gradually weaken with extended use. Although still very functional, this would make a rod seem slightly softer over time.
Today's graphite rods are mostly made with unidirectional (non-woven) carbon fibers and high strength epoxy resins. They can go through a million+ casting cycles without any noticeable change in action.
Over time, all rods are exposed to impacts (from weighted flies, branches, etc.), which can fracture the resin and lead to premature breakage under relatively light loads.
Does Lefty mean graghite rods? ...Chuck