ah, of course it have been discussed already https://discuss.rubyonrails.org/t/proposal-adding-postgres-p...
Since that discussion on the forum, I talked more about pipelining with some other core devs, and that may happen in some form or another in the future.
The main limiting factor is that most of the big Rails contributors work with MySQL, not Postgres, and MySQL doesn't really have proper pipelining support.