Example: I took the screen size of 1920x1080, I know what'll be shown in that particular screen's area when I stretch it to 1920x3000, there's a blue dotted line which shows the original fold's ending (1080 pixels) but after that there's no blue dotted which would show up after the end of the second fold (2160) and so on.
The pain here is, the user has to go inside 'Desktop Preview' or hit the 'Play' button to see what information is being shown in the first, second, etc., fold.

Solution: Just show a blue dotted line on doubling tripling etc., of the artboard from the original value. This will help in saving a lot of time and bringing utmost precision without hassle.

The fold lines appear, as far as I can tell, only when the artboard is chosen from the list of pre-made artboards. It has no "fold" lines on artboards that are custom sizes.

If I'm interpreting what you say correctly, you can already view the "fold line". When you click on the artboard you will see a blue dashed line across the artboard indicating where the bottom that respective device's screen ends. You would just need to extend the artboard to add any elements that call below the fold that users would scroll to get to.

Let's see if I can explain this. When you drag an artboard past the fold Xd shows a dotted line where the fold is. When I continue dragging the bottom of the artboard I would like to see more dotted lines every time I reach the next fold. Helps to see where the sections of a scrolling page sit.

dotted lines to show each default screen area with longer screens. Currently only the first screen area is shown with a dotted line. If my artboard extends downwards I want to ensure what design elements are available on each screen segment below this.