Back in the old (poorly named) "Oriental Adventures" 1e days, they broke down what was essentially Japanese armour into its component pieces. You could build the armour you were wearing based on those components, to yield a total AC. That's the only time I can recall that being done.
I like to think that if you're wearing a full plate armor in 5e you can convert it on a half plate or a breastplate unattaching some pieces.
There was also piecemeal armor rules in some 2E book. I forget which one. Maybe the complete gladiator's or fighter's book. I don't remember where, but I remember using it.
Right right, the authors have nothing to do with it.
