When I was coming up in the 90s what I found was it was mostly conversational, sometimes with some whiteboarding although of a much more relaxed nature than contemporary whiteboarding. It was more a tool to help express your thoughts and draw pictures than writing code.
There were some companies like Microsoft who were starting to become known for being more intense, but I never came across that.
It wasn't until the mid-aughts that I started to see brain teasers, "real" whiteboarding, and other aspects that started to bleed into standard interviews.
I'm to understand that Microsoft was one of the companies that had more relaxed interviews in the early 90s, and then took a turn for the more intensive in the mid-90s.
Perhaps the turning point was Jim McCarthy's "beware of a guy in a room", which he meant both figuratively and literally, leading to a transition away from solo or shared offices or even cubicles for software engineers and toward open-plan arrangements. I think that his wisdom led first Microsoft, then the rest of the industry, to treat software engineers not as the darling whiz kids they were in the past, but more a cohort to be closely watched and controlled, as the company is risking a lot on them.