Offline features for each game are still available, but the games can no longer be played online.
Ubisoft has shut down online multiplayer services for 90 of its titles, and player statistics have also been disabled.
For games that used Ubisoft Connect services, Units and Challenges will also be disabled, and this means you will no longer be able to earn Units by completing Challenges for the game.
You can still unlock rewards in particular games, but you will not be able to receive them in-game. Should your game have not transferred when Ubisoft moved from Uplay to Ubisoft Connect in 2020, all rewards will be automatically unlocked.
The update posted by Ubisoft also noted that unlockable content such as maps and skins will also be disabled, so you will no longer be able to unlock them.
On PC, unlockable content will no longer be available even if it has been redeemed previously. On console, it will continue to be available unless you reset your saved game files.
Games affected include Assassin's Creed 2, Beyond Good and Evil, Far Cry 2 PC, Far Cry Blood Dragon, Ghost Recon PC, Heroes of Might and Magic 5, various Just Dance titles, Prince of Persia: The Forgotten Sands, some older Rabbids and Rainbow Six titles, Rayman 3 HD, Rayman Legends PC, and many others.
Again, these are all older titles, many of which were made available on PS2, PS3, PSP, Wii, Wii U, and Xbox 360.
You can look over the entire list of games at the link.