Truth be told, you can get most of these games via other sources or in a physical form. In case they’re still available on Xbox One or Series X|S marketplace, they will remain there. Also, in case you already own them, you will still have access to them after they get removed from the Marketplace (via your download history).
The list of to-be-removed games are:
Aegis Wing
Assassin’s Creed Brotherhood
Assassin’s Creed III
Assassin’s Creed IV
Assassin’s Creed Liberation HD
Blood of the Werewolf
Blue Dragon
Breakdown
Call of Duty: Advanced Warfare
Call of Duty: Ghosts
Castle Crashers
Cloning Clyde
Counter-Strike: GO
Dark Souls
Darksiders II
DAYTONA USA
Defense Grid
Eets: Chowdown
Far Cry 2
Final Fight: Double Impact
Iron Brigade
Jeremy McGrath’s Offroad
Jet Set Radio
Left 4 Dead
Left 4 Dead 2
LIMBO
Lost Odyssey
Mass Effect 2
MONOPOLY DEAL
Mutant Blobs Attack
N+
Outpost Kaloki X
Peggle 2
Phantom Breaker: Battle Grounds
Prince of Persia
R.U.S.E.
Sega Vintage Collection: Alex Kidd & Co.
Skate 2
South Park: The Stick of Truth
Spelunky
Splinter Cell: Conviction
Star Wars: Battlefront
Star Wars: Knights of the Old Republic 2
The Orange Box
The Raven: Episode 1
The Witcher 2
You can find more information via Microsoft’s official support.