DY, Granted. My point was “a valid reason” is partly in the eyes of the beholder (I realize that is not what my previous post says at all).
Was it a real concern that somebody would ask for carrots and his friend though he menat yeast? Seems doubtful.
Or take a much better example. Kitniyos, the standard (unviersal?) minhag, among Ashkenazinm is to avoid Kitnioys at all. We treat it stricter than the chameshes minim which we do eat, obviously, as long as follows certain guidlines. Yet the minhag is to avoid rice cakes even if it just had rice and water, was shemira meshas nitiyaysan etc etc. This makes absolutly no sense.
I’m sure you or somebody else will come up with a “rational” explanation that it is a gezeira or a geder of some sort. which brings me back to my point. Rationality of minhagim is (partially) in the eyes of the beholder to me treating kitniyos as more chamur than wheat is completely irrational, to another it might make sense
What I meant with my original post was that minhagim based on a specefic societal concern, are (arguably) by definition irrational as minhagim (in the sense of the elevated status we give the term MINHAGIM as oposed to say mere cultural custom). Or put another way they arent really minhagim. LEt’s take a made up example. The town of hutzenplotz was very muddy so the minhag was to wash of their shoes before stepping into shul every day regardless of season. Should hutzenplotzers keep washing of their shoes even during a dry summer in a modern environment. I could hear a case made either way. and that is precisely my point.