AI said there is no equivalent.

 

The edit option in Windows Photos has a one-button "delete background" which can replace it with transparent or white (probably other colors, I didn't need, so didn't check).

 

Anything similar in Linux/Unix?  I needed this for an image with a monochrome portrait that had been superimposed on a colored background. 

 

If I could have eliminated everything not exactly greyscale, that would have probably been better (if there were a few greyscale pixels in the background, I could have manually removed them).  

 

Gimp would only remove one color at a time, and the background was "mostly" gold, but contained several yellow through red images.

 

A few web-based sites I saw charged fees, afaics.