Wish I knew how to answer that. I wonder if a wide angle lens would bring the steepness out more? Longer lenses tend to flatten, which reduces the feeling of depth.

Beyond that I am not too sure other than experimenting with the angle of the shot ie maybe not pointing directly up the slope but lifting the camera above your head and tilting down slightly, I think that would work with a wider angle lens.

Maybe somebody with more experience can help us...

Ad astra per aspera

climbing back down the wall of a disused mine

...in the Moravian Karst,...

*Traitor*, n.
A benighted individual who perceives an illusory distinction between serving his nation and abetting the criminals who govern it.

It looks pretty steep to me as it is. I would guess at least 50-60 degrees.  Maybe one just needs to use a little imagination.  Otherwise, you need to move/zoom wider where you can show more perspective to other parts of the landscape.  In this scene if you depicted someone climbing up above, I think it would be easier to visualize the angles involved.  We've all seen the shots of climbers either directly overhead or below, or clinging to a sheer cliff as shot from a distance.

I can swear there ain't no heaven but I pray there ain't no hell. _ Blood Sweat & Tears

I don`t know about photographing steepness, but I would suggest that you include something vertical in your shot (ie a tall tree on a slope shot from the side & across the hill, not up or down the hill.)
One more thing to maybe note.
 The human body & mind is wired to estimate steepness as much less from below, as from the top.
 This is a survival mechanism, & also offers a chance for the better dispersion of the species. It would be discouraging to climb a hill that appeared too steep.
The way the mind works is to instill confidence in exploring what`s on the other side &  with the perception of it being not too steep, the better specimen will venture forward. At the top though, the mind perceives the slope as steeper than it actually is, giving a more celebratory sense of accomplishment, a much grander perceived conquered step, & confidence in ones ability to tackle other problems. Thus a better person or animal to pass on better genetic qualities.
 That`s my theory & I`m now going to find myself a mountain to climb.

The difference between theists and atheists is that the atheists don't set the theists on fire for refusing to agree with them.

Having somebody falling would work - though maybe that is taking sacrifice for art too far :-)

Maybe it's because I'm a Londoner - that I moved to Nice. Blog - Nice Experience