AnyDesk sets up its autostart setting bypassing KDE's autostart configuration (which involves creating a .desktop file in $HOME/.config/autostart/).
On the contrary, AnyDesk creates its autostart by creating a systemd service, which is a lower level process (therefore the DE's autostart menu cannot see it). It is not a "hidden autostart app", but here the kernel starts the process automatically.
To stop AnyDesk's autostart, either disable it in the AnyDesk settings, or disable the systemd service with the command
sudo systemctl disable anydesk.service
What about other desktop gui like Ubuntu or Most Power-Efficient Ubuntu Mate, where is their autostart apps hidden?
Since this is not controlled by the desktop environment, the solution remains the same.