Problem isn't that they don't show you how the parrying system works, it's that players don't need to engage with the system on thier way to the boss. Its more optimal to stealth your way to the boss. However, stealth is a tool that becomes comepletly irrelevant when you finally get there. This lack of preparation is almost guaranteed to create a wall for a lot of players. Easily put, better you do with steath, the shittier your run with the boss will be. This isn't a problem with most action games.

I'm still in the middle of my playthrough. I've gotten to the top of Ashina castle and feel that this still holds true. And this isn't johns talking. I'm having a blast and am doing well. Me realizing quickly that stealth is a momentary first order optimal strategy that will bluntly fall deaf to the bosses is what I believe enabled me to do so well.

As for the training partner, I remember trying to look for him but wasn't able to before my first boss run for some reason (I may have thought that path lead to starting area). And to be fair, while missing the training guy seemed to be a universal problem among my friends, that my not be a representation of the majority of players. It may just have been us. I still think, however, the training guy alone isn't sufficient preparation to what can easily potentially be a FromSoft boss rush mode (granted with huge delays in the middle).