I didnt pick 20 years to "measure" our knowledge of football. I picked that timeframe because it was about when the NFL started to shift the rules about what you can and cannot do to WRs.and I was watching football before you were born as many games as you have. I saw that first hail mary that Roger Staubach through also.
Even in the modern NFL there are 2 times when you will almost never see PI called. One is when there are a bunch of bodies all going for the ball, which is most commonly in a hail mary but also occurs when you throw it up into triple coverage and all of them are around the ball. The second, though not relevant here, is when you can argue both the WR and defender were committing PI. You almost never see offsetting PI. Generally when both sides are shoving each other they will just let the play go.
Now the second obviously does not apply but in both cases nowhere in the rulebook is that stated but yet in both cases you know that those are the rules.