Hello,
On Fedora Workstation 42 I encounter this issue that brightness slider is missing but after I use physical keyboard keys to lower/higher brightness level then brightness slider will show up!!
I also made a new user and checked with it but issue not related to user settings or even gnome extensions.
Before pressing physical keys:
After pressing physical keys:
I just checked, and I have the same issue with Intel graphics. I don’t remember what the behavior was in 41, so I can’t say if it was the same or not.
Since I always use the keyboard to adjust brightness, I didn’t notice this until you brought it up.
Working out of the box with KDE 
@litemotiv mentioned that issue has been fixed and will be back to Fedora but takes some time:
On my Dell laptop, the brightness control was missing from the top bar in Fedora 42. I was able to fix it by updating the system firmware with the following commands:
sudo dnf install fwupd
fwupdmgr update
fwupdmgr get-updates
fwupdmgr refresh --force
fwupdmgr update
After rebooting, the brightness control reappeared and is now working fine.
