Can Microsoft support let me purchase redeeming digital key codes for delisted games?

Microsoft cannot legally provide you with his games, as they no longer have the right to sell them and frequently neither do the developers. Games are not delisted for fun

Valve delisted of the games, I wish you luck attempting to get them to do anything

No one here would know that. You can still try to find it on disc and play it that way.

Well... is there any chance that Left 4 Dead 2 might get remastered in the near future? It's still a beloved game and lots of other players on xbox still play even without the dlcs. If Microsoft ever made a remastered version for xbox one/series x/s I would be more than happy to by one.

No. Once they're delisted there's no way to purchase them digitally and support cannot bypass that.

Hello Zombiekillerr97, this is Manuela. I'm an Independent Advisor and community user like you, ready to assist with this.
Not a Microsoft employee.

This game has been delisted from the Microsoft store by the game developer a couple of years ago.

Unfortunately, the only way to get the game digitally is by purchasing a redeemable key, if available, from a third party store.