There are a few ways to achieve "this person walks through the wall" in 5e; and I suspect the exact mechanics aren't too important for the question.
Suppose I have an enemy backed into a corner, in combat, and they respond by walking through the wall ... do I get an OA?
In particular, if I further have Sentinel, am I able to prevent them from walking through the wall in question?
Is the answer any different if the movement is "sink through the floor"?