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 a 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.