Much of the deployment of OAuth2 was a shitshow, because early on, there was a huge debate about OAuth1 vs OAuth2, and the concepts and terminology used in the OAuth2 spec exceeded the mental load of people who were simply looking for a quickstart to some API. Docs by various providers either adopted the OAuth2 terminology wholesale without sufficiently explaining the new concepts and the best practices, or they simplified the client registration to just a few fields and offered no guidance as to make use of them in your app.
The spec has been clear from the start that there are clients that can protect their secrets, and ones that can't [1]. Here's a choice quote: "A native application is a public client installed and executed on the device used by the resource owner. Protocol data and credentials are accessible to the resource owner. It is assumed that any client authentication credentials included in the application can be extracted." Unfortunately, much of the rest of the spec is a bit like a playing puzzle, where reading it in one sitting doesn't actually build your understanding, and you have to constantly cross-reference various sections to construct a complete picture of the process the spec is trying to set.
It look a long time for people's familiarity with the spec to grow, and for providers and clients to improve in quality and adherence to the ideas and text of the spec.
[1] https://tools.ietf.org/html/rfc6749#section-2.1