This is part of the Semicolon&Sons Code Diary - consisting of lessons learned on the job. You're in the bash category.
Last Updated: 2023-03-28
I attempted to run this (ill-advised) code as an experiment:
ssh user@ip -i /dev/stdin <<< "THE PRIVATE KEY"
It didn't work for various reasons, but one interesting one is that it isn't possible to have an interactive terminal when STDIN is already spoken for, as it is here. Think about it: if STDIN is already spoken for, how can the SSH prompt receive input?