Detonation depends on both CR and combustion chamber geometry (and other issues such as inlet air temperature/pressure and coolant temperature), plus different manufacturers have different philosophies, and they're always concerned with warranty claims.

Modern cars have detonation sensors with multiple spark advance maps. If an engine sees a lot of detonation due to low octane fuel it will default to a less aggressive map. This will usually result in less low end torque/torque power that most users probably won't notice and maybe high fuel consumption due a less than optimum spark advance map.

Some owner's manuals might say "91 recommended" rather than "91 minimum", but I doubt if most recognize the difference in these two statements.

Bottom line is that most modern engines will operate fine on 87 PON despite what the manufacturers say, but it's up to individual owners to test and determine at least SOTP any difference in performance or fuel consumption and listen for detonation.

Duke