Years ago there was no mechanism to produce food before Pesach and to preserve food for Pesach use. As a result, most foods had to be produced by the consumer at home on Pesach or right before Pesach. Many of minhagim of what was not eaten on Pesach is a direct result of this reality and has absolutely no bearing to the current way of producing and processing foods and the sophistication of the hashgacha enterprise. Most of the non-meat foods did not have hashgacha all year, so for Pesach many of these foods were not used at all.
Our housewives are not opening up and cleaning chickens and gizzards right before Yom Tov and our floors are covered, vacuumed and chemically cleaned.
Our homes are more halachically cleaned and hermetically sealed than the homes of the past. Many families are already pesachdic a days before Yom Tov, which was never the case back then.
It was not economically viable to produce dairy for just the few days of Pesach since everything had to be fresh and, no matter what, dairy was not eaten on Yom Tov or Shabbos.
So it seems to me that while there was good reason at the time not to use certain foods, today I would consider them shtus minhagim; certainly those minhagim that were not universally accepted by ashkanazim or s’fardim as a group.