What is the ruling on using highlighters in the Mus'haf to recall certain verses, and what is the expiation for that if it is unlawful, especially for one who did so out of ignorance of the ruling?

Highlighting verses in the Mushaf with a fluorescent pen or similar instruments is not appropriate, because showing respect and reverence for the Mushaf is part of revering the rituals of Allah. The obligation to protect and respect the Mushaf is agreed upon. This highlighting may lead to diminishing the sanctity and awe of the Mushaf. The intended benefit from such highlighting can be achieved by placing a sticky note to mark the verses.