How can one still read words that have been crossed out with pen ink or magic marker?

Gosh, yes! Hold the paper up so your viewing angle is more from the side than above. Tilt back and forth until the light angle is somewhat slanted. See? When I cross something out, I use the same pen, I use straight lines at various angles, then lots of circular motion after that, like a string of cursive “e“s.
Still, when someone is writing, their movements are deliberate. Crossing something out, the movements are more casual. Spotting the difference is not so difficult.

There is black, then there’s black, and both are different from black. The differences are subtle, but they’re different enough to remain legible. And if the paper got dented by the writing/printing process, the differences are even more obvious.

The only way to ensure that someone can’t read lines of text on a page is to physically remove them with an x-acto knife. Differences in inks can be spotted, and electronic redaction is ridiculously easy to undo.

We need to know if you’re trying to hide something, or find something hidden.

Repeatedly covering something with scotch tape and removing it will slowly remove the top layers, eventually revealing the original post, most of the time.

Do you have time to enrol in some course for background workers in the secret services?
You would have a head start on others on the course with all the helpful answers given here!