So 200, not served by proxy cache. That page is set as “Archived” in Matrix.
As for “_”, I don’t know, I’ve inherited that system when I started working here, I’m not sure why they did that but I’m assuming it is a valid character in URLs so unless NGINX or Matrix has a RegEx excluding that from remaps it shouldn’t be an issue, worth investigating though.
Oh, if the asset that the URL is pointing to is still in the system, the remap won’t work. Especially if you are viewing it as a user who has admin rights to the asset. You need to remove the web path off the archived asset so that the URL doesn’t point to any asset at all and then the remap should work.