FEZ. It’s only playable on Xbox 360 as it was never made backwards compatible. Actually think I’ll boot up the 360 and play it next. 

Excellent mention.

Fez is so bizarre because it's available on

-Windows, Linux, OS X

-PS3, PS4, PS Vita

-iOS

-Nintendo Switch

And of course, Xbox 360 which is the OG version. I suppose Fez isn't backwards compatible or native on the Xbox One due to licensing/financial issues. 

You can go into your purchase history on your Xbox 360 and download from there.

Keep in mind that same issue still exists as there is a web Xbox One/Xbox Series store and a web Xbox 360 marketplace.

See here for Xbox One/Series:
https://www.xbox.com/en-US/games/all-games/#

And Xbox 360 Marketplace:
https://marketplace.xbox.com/en-US/games

It had a ton of Xbla titles but not sure whats b/c.

Saboteur is a nice one to collect.

@Pemalite 

So, if I'm interpreting you correctly (which this is useful information for anyone wishing to buy games/content for Xbox 360 before the digital store closes)

-Games will appear on your download list if you use the Xbox 360 marketplace pages or of course buy them directly on the Xbox 360.

-Backward compatible games won't (or are inconsistent) appear on the Xbox 360 download list if you buy them on the newer consoles or newer webpages/apps. It's a shame if this is true, because there are some great deals sometimes. 

I also understand that you can't add redemption codes on the Xbox 360 anymore for digital or physical gift cards. 

Correct.

When we had Games with Gold XBox 360 games (R.I.P), I always "redeemed" them on the Xbox 360 or Xbox 360 web marketplace and could push the download through next time I boot my console.

Redeeming codes on the Xbox 360 is still possible.

You need to redeem them on the console itself or on the Xbox 360 marketplace, often if you try to redeem them on an Xbox One/Series console you just get an error, they are physically two separate online stores.
Microsoft needed to market the differences better as it is confusing, but with the Xbox 360 going away it is probably irrelevant.

I added a list to the OP. The list is pretty accurate from what I've heard.
Some other oddities I wanted to bring up outside of the OP.
Kung Fu Panda: Showdown of Legendary Legends was never delisted from Xbox 360. It's the only digital version of the game still available, but it's a physical game too. More interesting is that it is the only way to get the DLC for the game.
Snoopy Flying Ace (a digital-only exclusive) was delisted a long time ago, but still has a free demo. There's no other official release anymore.