As you might know that Nintendo announced that “In early April 2024, online play and other functionality that uses online communication will end for Nintendo 3DS and Wii U software.”
I think this video is a very helpful list of all the games the person found that will be affected by this, giving a brief description of what will be lost for each.
Some games give free single player content, so if you want to complete the games 100% it is best to start now and finish before the server goes down. I’m going to list those games below. Watch the video for more details on these games (or the other games I’m not listing because they don’t impact completionists).
He lists the following games as losing something that “impacts completionists”