ok, i checked the ENTIRE area in and AROUND where this is, and i didn't find an entrance/exit, so I'm pretty sure that this has a meaning.
the picture explains most of it, but ill try to say whatever is needed.
alright, as you can see in the picture, lines. follow the lines. just regular jump like you would in a corner to get up the first line, then wall jump up the steep hill.
its as simple as that. and as for getting out, just more wall jumping, but just in case you cant do it, make sure your hearth is on cooldown. (unless that one exploit still works that lets you use auto unstuck to hearth again.)
BTW, the "first line" refers to the line that has the arrow right after it starts.
EDIT: ok, i just noticed that there is a exploration section that was created during the time i didn't check it. so, if you feel like it, move this post to the exploration section if its needed.