Open or closed it rarely matters because OKW has no reliable spotting capability for the JT.
Send in spotters -> spotters get wiped -> JT alone without support = dead JT
Have units cover JT -> Nothing to spot for JT -> Enemy armor roams free and you lose map control = lost game
Best way is to send your infantry or any Panther forward, try not to blob the volks in case there is an ISU or to be a big target for artillery. Slowly creep the Jagdtiger forward and snipe the tanks that have revealed themselves trying to shoot your infantry. If this is 2v2 and up, it will be easier for buddies covering your sides or meatshield for you.
I agree that it is a pretty significant change,and it will take some time to adjust, but it was very OP imo before. It could snipe tanks while hiding out of view, offering very limited counterplay, especially with the amount of good units OKW and Ost have.
I am still skeptical about giving the Jagdtiger something to compensate for this nerf. The unit is still devastating in open areas (even without the obstacle-shooting ability), so adjusting costs and stats would make it even more difficult to deal with.