Hi Alan,
The Ctrl+Z absolutely still exists in Windows 11 and it works in Word, Excel, Notepad and most other apps. When it does nothing, the most common cause is that Windows switched your active keyboard layout, so the physical key you press is not sending the letter Z for the current layout.
Please try this.
Press Win+Space once or twice and watch the language indicator on the taskbar change to the layout that matches your physical keyboard, for example ENG US or ENG UK. Open Notepad and type the letter Z to confirm the key sends Z, then press Ctrl+Z to undo the last typed character.
If it now works, make the correct layout stick so it does not change again.
Open Settings > Time and language > Language and region. Under Preferred languages, beside your language click the three dots, then Language options. Under Keyboards, keep only the layout that matches your hardware, for example English United States - US or English United Kingdom - UK, and remove any others like US International or a different language. Close Settings and test Ctrl+Z again in Notepad.
If that does not sort it, tell me what layouts you see on the taskbar and what happens when you press Ctrl+C and Ctrl+Z in Notepad, and I will guide you further.