Watching videos of how to beat this boss, all of them from two years ago, I started to think: FS patched the game to be harder. In the videos, players spams attack button, Owl (Father) does nothing. Try to do the same, he jumps, he evades, he kicks your face and says you’re starving. Nine necklaces, he kills me in two hits. YouTube guy, life bar by the half, takes 3 hits in a row, not blocking or deflecting, still alive - no use of any candies or spiritfall. And sometimes Owl stops attacking, he just walks in circles for a while. Never happens on this side: he doesn’t get off me.
It seems like FS noticed gamers were finding different strategies to progress, identifying AI weaknesses and taking some advantages. And then they took it off. If playing aggressively was fun to players at first, now they made Owl more aggressive, breaking their own logic. He has smaller safe windows to hit him and almost all of his attacks can't be evaded - they track you.
Evading and deflecting - basic mechanics of the game - don’t work here, and when I beat him, I don't felt like I "git gud", as you say, I felt it was pure luck.
Don’t get me wrong: Sekiro is a great game, and has some bosses that were very fun to beat, but when the devs fix things just because players are finding it easier than it was planned to be, this is just dishonest.