Buying a code for a delisted version of a game.

Sometimes they do and sometimes they don't. There's no guarantee it will work.

It really doesn't make much of a different. The physical and disc versions run the same. Even if a game is delisted, if you purchased the game before it was delisted, you can redownload it and continue playing. If you didn't buy the game before it was
delisted, then you would need to find a physical disc as that would be the only way to purchase it.

You can as long as the game isn't delisted from the sounds of it it's delisted

What game is it

If you are buying old codes for the sake of old games I'd just get updated games as you can't refund old games

Currency codes yes

Game codes if the game is delisted you cannot make new purchases of a delisted game

forza horizon 1 was delisted a long time ago now, to my knowledge 360 games that are delisted redeem codes will not work, now delisted xbox one games work with redeemable codes.