I've often wondered the same thing. My girl Kiwi has never gotten rid of a solid line that goes across her whole body. I'd hope after the length of time we've been together she isn't constantly stressed. They aren't vertical bars, just a horizontal line from head to tail. And she's had it since day 1 I bought her.
Lol Lucy it was fine but this is better to get noticed. To give my IMO Marbles can be this way. Some marbling I've seen on bettas has been from dots to lines. But usually marbling will change or go away over time. Other then that I'm not sure....someone else might have a better insight then me.
I would say that is marbleing. I'll post a picture of scropio when he is stressed next time I do a water change and give you an idea of what I mean though I'm sure MM, Critter or Whisper could post one before me lol
I don't think that the wild type betta spelenden females have permanent stress lines... They are rather dull brown with the typical short fins but the only "line" would be where the iridescent dots that go down their sides. I have a couple of females who stress easily but the lines still weren't permanent.