It was probably about as hard to move between those domains now as it is today. Which is to say that it's pretty hard and needs some concerted, non-trivial effort in shaping your experience and how you present it before trying to make a transition, and often either some kind of inside reference to vouch for you or an employer that was especially hard up for candidates. Or else an employer that straddle multiple domains and actively supported internal transitions.

Depending on what you could bring attention to in your prior experience and the size/needs of the new orgs you seeking to move to, certain transitions were more feasible than others, but you could easily spend decades working in mind-numbing enterprise applications while wishing for opportunities in game development or trading or whatever and never get your resume so much as looked at. (And vice versa, even, for those who dreamed to "retire" into the supposed quiet of enterprise apps or government IT or whatever)

I basically agree with your edit. There was a lot more fluidity among roles and even just moving into computer roles from other engineering (and even non-engineering) fields. But that's not really what you wrote initially.