I'm asking for specifics because I actually can't think of any apps where "undo" would make sense, but is missing. I'm probably not creative enough. But that's why I asked for examples. These are the things I thought of:

Text editor? Those have undo.

Image editor? Also have undo.

Video Games? Depends. Should you be allowed to "undo" in a game of minesweeper? Probably not- it kind of ruins the game. Should you be allowed to "undo" in Chess? Probably not. Should you be allowed to undo in Solitaire? Maybe.

Chat client? What would you undo? Can't unsend a message.

Web Browsers have back buttons, to the extent that those can undo.

I could see file managers maybe having undo. I honestly don't know if the popular ones today do or not.

What apps are missing undo functionality for you?

Sure, but the text editor does have undo.

Text is important, but its datum is rarely independent. More undo is useful than the text in the current text control. Anything else requires (and rarely gets) more substantial command undo/redo. I'm not making a black&white point, I am honestly surprised it so easily became "non-essential without apology".