I don't know what stock you mean by "treated deck boards" to get 2"x8', whether 2x's or 5/4 decking, but ripping them you'll have an eased edge on one corner. In a design where you are trying to hold very strict tolerances, you'll go nuts trying to find straight enough boards without knots, and nuttier once they start to dry and shrink in ways you can't control and hold those gaps. If the inspo pic is what you are replicating, you'll have to use a high grade of wood, otherwise using treated is fine if you lower your expectations.

Thanks for your response. I was thinking of ripping 5/4 treated into slats then dark stain. My only concern was warping and twisting with 8' lengths. But I am now considering 5 foot wide sections instead. I would be happy if it lasts 10 years. too optimistic?

The 2nd pic, which is very different from the 1st pic, would be more successful with your plans, though with each plank ripped at ~3" wide spanning 5' I would just be guessing if it would sag or twist (the full 6" wide would be fine).

8’ Span will not hold up, nor will 5/4 deck boards really. My fence is 2x2.s ripped from 2/4 pt. Posts are 4’ on center.

Untreated wood, in the ground, lasts about a decade here.