As I mentioned above, the twistweights of modern rackets have gone up. This gives that subjective "solid" feeling. They also have equal or higher swingweights which is achieved with less head light balance as that's easier to manage for most players across most skill levels because the overall weight can be kept the same by shifting it towards the top.
Stiffness seems to go up or down depending on the respective year of the same model. I didn't know about any of this when I was younger, I just knew I didn't like these types of frames and kept my 6.1 because it felt better. But that's because I knew where the ball was going to go due to the head light balance, not crazy high twist high (less "solid stability" as TW reviews call it), and a flexible frame. There were much stiffer rackets even back in the 90s, the Wilson Hammer lineup was ridiculously stiff with an RA into the mid-70s. "Plowthrough" is another subjective term which doesn't really mean anything, it's a result of swingweight and twistweight making it feel like the racket does the work for you which I have definitely felt on a lot of modern frames but not on the 6.1 95. You have to put in your own power which is what makes it more of a reliable tool. Not quite as low powered as the 90 or 85, but certainly less than a modern Babolat. The RF97 also comes to mind, same headlight balance but feels much much heavier on contact than the 6.1 95 due to the higher twistweight. Then there are things like recoil weight, beam shape and thickness, and other technical bits which are quickly outside of my wheelhouse but those also come into play.
The 6.1 95 has a wonderful feel as I mentioned above, but it's not an immediate and satisfying "solid plowthrough" feel which you get on a modern Babolat which is why I suspect it didn't sell as well for most recreational players. Most players are not 5.0 or higher and aren't going to enjoy or utilize the low powered nature of the 6.1. Which makes me sad because I need more rackets and grommets!
Twistweights
KSix-One 95 (18x20) 12.9
I've hit with each of these this year and these stats feel in the ballpark:
Pro Staff 97 RF Autograph 14.9
VCORE Pro 97 (330) 14.23
Pure Aero (2019) 15.36
Pure Strike 16x19 3rd Gen 14.76