Even as late as the late 90s, probably into the mid 2000s for manuals. The complexity of vehicles from that era is mostly in understanding the role of the sensors (or that they even exist in the first place). Once you understand that, they go back to being pretty straightforward. I guess the exception here is probably immobilizers.

The complexity of modern vehicles comes down to software coupling between the systems, which (to my knowledge) started with automatic transmissions, and then spread to everything else. In very modern vehicles, even the sensors register with the ECU and communicate on encrypted channels.

The reason for these measures is ostensibly security and quality, but automakers don't seem to mind the fact that it also makes many repairs more difficult or impossible.