It's just one of those unspoken rules, I've found it to be prevalent in every gaming circle I've come across. Tailoring to a specific opponent is a nono. Writing a list that fares well against a certain army configuration is okay though.
For example, during 5th edition I favoured xenos lists heavily, primarily tyranid and necron. My preferred list build was always in favour of numbers, so I ran a swarm of gaunts and a horde of necron warriors in each respective list. My opponents knew this, and most of their armies adjusted by carrying more flamer weapons, and two of my marine player opponents went from fielding one vindicator to two.
It gave them a sporting chance against the numbers I fielded without being a complete counter.
So all in all, I would say tailoring to a specific army typeset is okay, tailoring to a specific army list is not.
Nonsense is our Salvation