Speakers hung from ceiling on aircraft cable.

Opinion question:

We are thinking of suspending a pair of KEF LS50 speakers from the ceiling using aircraft cable. Do you think having them not rigidly mounted will diminish bass? Will the woofers cause the speakers to move?

Surely a flat platform with a filament at each corner would work, kind of like a child's swing, only don't push it!  Blu-tack speaker to platform.
Newton's third law says no.

The drivers are motors and pushing back and forth against the cabinet. If they are not rigidly held in their 3 dimensional space relative to the listener you will introduce all sorts of artifacts.

In fact, even with stand mounted speakers, mass often helps to improve clarity and performance but your suggestion is even worse than a basic stand mount.
I wonder if there would be a correlation between people who tell you suspending speakers from cables is a bad idea and those that tell you to put your speakers on spring platforms?

In terms of losing bass, just remember that most concert speakers are suspended.

It would also do to research the relative weight of the cone+voice coil compared to the total speaker weight.

In terms of mass in cabinets, that typically improves rigidity.
When I was about 20 I had my speakers suspended with some hi test fishing line. It rawked dude! Probably not something that I’d do now.

It might actually sound okay. Worth a try, especially with some relatively small speakers like those.
You could try mounting them on a ceiling mount pole designed for TVs. Or something like that.