Hello!
I've been having this issue and held back on updating my HP ENVY x360 Convertible because every time I updated to the 23H2 build, my brightness slider went missing and every time I reached out to support, they eventually just had me rollback my update to where my brightness was working.
TODAY, I have found a solution! I just wanted to share in case anyone else with a similar issue is looking for an answer.
After updating to Windows 11, Version 23H2 build, if your brightness slider is missing and/or you can adjust your screen brightness with the buttons on your keyboard or with the slider, follow this and see if it works:
Windows Key + X > Device Manager > Monitors > Right-click on "Integrated Monitor" > Update Driver > "Browse my computer for drivers" > "Let me pick. . ." > select "Generic PnP Monitor" > Next
Hopefully this helps for anyone who might have this similar issue!