The cue I learned for that was "the stick man is standing on the line | and leaning forward / or backward \ " (Which is of course western reading direction sensitive)

I always remembered it as, apparently they are both handwritten as an upwards stroke. Assuming western LTR reading direction, it means the slash forward goes forward, and the backslash goes backwards with respect to the reading direction.

I mean, I never really thought about it, but that's how I "know" which is slash forward and which is backslash.

I think that's mostly due to inconsistent terminology, with maybe a sprinkling of windows file paths. I think if most people used sed for a week they'd figure out which one goes between sections and which one escapes characters pretty quickly, even if they didn't know which one was the "back"slash.

Slash and back Slack. [0]

[0] https://static.fjcdn.com/pictures/Slash_066306_2216065.jpg

the reason for the name of the slashs is: if they'd fall, do they fall forward or backward?

/ forward slash

\ backward slash

For me, “forward” and “backward” are not applicable to the text, because they are in the different plane. Forward-backward vector lies on the line of sight, that's orthogonal to monitor or paper/book.