Features definitely were not a huge part of "golden age hip-hop" of the late 80s.

Even that stretch in the early 90s didn't having many features. Enter the 36 had none, Ready to Die had one (Method Man), Illmatic only had an unknown AZ, Reasonable Doubt had a few but outside of Biggie it was mostly guys from Jay-Z's immediate crew.

I'm not really sure when that trend really took off, but it is an interesting point.

I would guess that the establishment of looser crews, like Snoop and Dre and the Dogg Pound, and Jay-Z and the Roc-A-Fella group probably started it, and eventually it turned into completely unaffiliated guys jumping on each other's tracks.

It's also worth pointing out that until Illmatic, production was usually handled by one producer.