These exist. At least the "on/off" part. Look for "switchable mirror glass."

curtain

Could easily be done. We have glass that can go opaque in response to an electrical charge. Mirrorize a pane of that and you have a mirror that will go opaque on demand.

A two way mirror where you turn on a backlight behind the mirror to turn "off" the reflection?

It could be a nice painting or something behind it?

https://www.kentoptronics.com/mirror.html

I could use this for my magic mirror. Which I really do have and would like to turn off on occasion.

It's a wide mirror in the back room that you can see in the hallway. There must be an imperceptible curvature to it because it makes me look pretty buff.

The thing is, I know I am old and out of shape. It's just this one mirror. I look normal in other ones.

Call me jaded, but I think the way it would actually work would be:

Budget Version

On: Normal mirror (with ad in the bottom right hand corner)

Off: slideshow of ads

Premium version

On: on

Off: off