I believe this is retro wave flat

Same

I found the only working version of this terrain I could, but without the retro sun/skybox + the terrain is more mountainous/hilly.
I still hope it helps
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2070353973

Me being 2 years late hi, it's a CS:GO AWP 1v1 map, it has a pretty huge out of bounds area that you can walk around in, i just put the folder of it in my addons folder and it worked, you won't find it on the workshop