A few years back I picked an early 90s Reynolds 531 road bike with Shimano 7 speed which weighed about 10.5kg
My
Halfords Carrera Vanquish with Sora 9 speed weighed about 10.5kg
My 1991 Dawes Reynolds 500 MTB weighed about 14.5kg
My 2015 BH 29er weighed about 14.5kg
So like for like i.e. entry level stuff, the wights have not changed much in 30 years.
I think if you go above entry level there is a difference.
If you compared a Reynolds 753 pro bike from the 80s with a modern Pinarello Dogma race bike there'd be quite a difference.
From what I can gather the Dogma frame is about 1kg lighter at 820g compared to about 1800g for the 753.