>>200274980s mtbs have more relaxed, upright and slack geometry, having evolved from 3 speed cruisers and touring bikes.
90s mtbs, high end ones, are longer and lower and usually specced for roadies to ride fast on gravel.
The iconic classic shimano groupsets came in the late 80s. Those gave modern indexed shifting and decent braking.
By the mid 90s you had STI shifters, suspension forks, bonded forks, aluminium frames and v-brakes, and this technology is more prone to wear out due to age than thumb/downtube shifters, rigid steel forks and steel frames. And if you want v-brakes you want disc brakes. This late 90s mid-school stuff has been largely made redundant by the modern bikes that superseded it, whereas older early 90s / late 80s stuff has a more valid vintage niche because the gear is more durable and it's less race oriented.
It's not 80s vs 90s though, it's more like 86-95 vs 95-03
