Unless mistaken, I don’t think that Okular includes a ruler to help reading by highlighting a line, like the Sioyek visual ruler.
I have personally found this very useful in the past, and if such a feature does exist, please let me know how to activate it.
Otherwise, I think that adding such a tool would help people with dislexia, reduce eye strain, and continue the ongoing work to add accessibility features to KDE software!
Can you post a screenshot of what it looks like? Is it just a highlight or does it hide other text not on the line you’re reading like a physical ruler would?