I just learned that there are programs to control the brightness of external monitors just like you can adjust your laptop’s integrated display. On windows, the most well known one is monitorian (FOSS), on linux you can (on Gnome) even use shell-extensions to have a brightness slider just like you do for the integrated display.
I might be out of touch, but is this well known?
They’re typically not a physical toggle though, but a simple board which passes a message to the lcd controller board. You can also send these messages via your display connection.