Fatigue is a different thing than increased flexibility (called whippiness in the vernacular). As a few have pointed out, frames don't develop flexibility. The fact that they can fatigue means they can BREAK but they don't get flexible over time unless there is a crack which is another matter. Stiffness and strength are different things.
philbob57 (the original poster) is asking about flexibility, not about breakage, so the topic of fatigue here is irrelevant.
I remember in the 80s when the urban myth spread widely that frames got "whipped" or even "whooped." Even a couple of bike shop owners told me this.