Great thread and I'm also interested in the rationale behind the 1989 cutoff as I'm sure that could spark further debate.
For what it's worth, and I suspect not a lot, my interest in road bikes is mainly 1984 to 1992. Why? For me personal resonance is king and there is no family interest in cycling so I can't easily connect to another, earlier era. Therefore I'm sort of stuck, happily, with an age that has meaning in my life. As an aside, this is a splendidly egocentric approach that also applies to music, films and literature - for those I'm 1997 to current day. Anyway, I digress.
Fortunately the 1884-1992 period was a classic
era for the Tour de France - we had Miller, Lemond/Hinault, Roche/Delgado, Lemond/Fignon and the emergence of Indurain amongst others. Couple that with some iconic team kits (La Vie Claire, Hitachi, Panasonic, PDM, 7-11) plus the end of Super Record and the birth (and death) of C-Record and it's arguably as good an 8 year stretch as any other (although I'm sure very persuasive case could be made for other eras, and that's at least half the fun).
I'm glad that there is no formal definition as to what constitutes a retro road bike as I think that would be rather limiting and the diversity on here is definitely an attraction - if it was only 84-92 there's no doubt that I could and would learn something but there's far more for me to gain when other eras are included.
Enough of the analysis for now, part of the joy is to respond to a bike on a visceral level without overthinking - do you feel it? If the answer is "yes' then it's all good (although I must acknowledge that the restraint involved in not stating 'vive la difference' was considerable!).