I found out that itch.io is missing app authorization to find out if users own the games actually. What I want is to find out if players own my game. And if so, they can play the game. itch.io is missing that thing, so it's easy to get pirated for our itch.io games.
As in contrast to which games? If you can get pirated copies of AAA games, what do you think your games are?!
DRM is a problematic concept in many ways. Foremost, that it only punishes legit players. You will not sell more games, because a game has drm. The creator might have the fuzzy warm feeling that the game is harder to copy. But the concrete cold reality is, that no one will buy the game because it has drm, but many will intentionally not buy it, if it has one. Just read some steam reviews of games that implement additional drm.
For any kind of online experience, drm is moot anways. You have your servers and login credentials, a thing like client side drm does add nothing.
The api you mentioned requires you to have a server. Wich brings us back to the online experience. And there are users that will be very, very suspisious, if an offline game requires internet connection. And quite frankly, I would not trust a small time indie dev to handly any such thing at all.
But from what I understood, you could implement some login type drm with this oauth thingie. But after having a look at your creators profile and games, your concerns about pirated versions of your games are purely academical. And to be more specific, I for one would not buy any of your games, if I had to login onto your servers to play them. For the single reason, that I do not trust a small dev like you to still have that login server in two years.