You'd think every DAW would have something like this: Subtract everything that's stereo (AKA keep only the sound that's present in both channels).

I have old mono records that I wanted to clean up. In that case, any stereo content is obviously scratches and surface noise, so removing it would be most of the job. But nope... not one DAW offered this filter, despite offering the opposite (removing mono content and keeping the stereo).

And yes I did try removing the mono content and then subtracting the result from the full source, but this didn't work; I don't remember (or know) why.