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.
It's called the DDC protocol by the way. Like someone else mentioned, Twinkle Tray is a great option for windows, as is ClickMonitorDDC if you don't want to use windows store apps.
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.
One of the sticking points for linux wayland nvidia drivers, can't actually dim the monitor down to a sane level for a dark room. Sigh. Marketing drivel about hdr brightness nits but no data on how low can you go...