I downloaded a multitrack of Michael Jackson's Billie Jean. The backing track is full of FM synths and subtractive synths. If you solo them all, it's like listening to MIDI file on an old Sound Blaster card. If you solo everything else, it sounds like a 1970s RnB track with missing parts. It's everything they didn't use synthesisers for that really defines the track. I suspect that's the case for many tracks from the 1980s, despite popular belief and vaporwave nostalgia.