There has always been a lot less hard sci-fi than soft, true, and it used to be more common than it is now.
But the real dividing line in my mind isn't quite as stark as I made it sound. There's still a gray area. Unusual events that exist purely for plot purposes don't disqualify anything, for instance.
The differentiator I have in mind is more basic: if the story involves things that are simply not possible, it's not hard sci-fi. If it involves things that are very unlikely, but still within the realm of possibility, it can certainly still be "hard". Same if it involves things/effects that don't (as far as we know) exist, but wouldn't break the laws of physics if they did.
Off the top of my head, I'd count Asimov's Foundation, Crichton's The Andromeda Strain, Niven's Ringworld, Gibson's Neuromancer, Bear's Darwin's Radio, and Stephenson's Snow Crash.
Those are literally just morality plays with no scientific grounding whatsoever. The "positronic brains" behind the robots only exist because Asimov thought it sounded cool and futuristic.
But the real dividing line in my mind isn't quite as stark as I made it sound. There's still a gray area. Unusual events that exist purely for plot purposes don't disqualify anything, for instance.
The differentiator I have in mind is more basic: if the story involves things that are simply not possible, it's not hard sci-fi. If it involves things that are very unlikely, but still within the realm of possibility, it can certainly still be "hard". Same if it involves things/effects that don't (as far as we know) exist, but wouldn't break the laws of physics if they did.