
I'm not able to move and I'm not using the noclip cheat...
The location is the right side of the first bridge in the Badlands (Map 11) that faces towards the waterfalls. Once entering that position, you can't move out of that position unless you cheat. You're "stuck" to the bridge wall.
This doesn't affect ZDoom 2.0.98. (It is GZDoom specific).