Slots isn't a messaging system in the way Objective-C or Smalltalk define it. Slots are what GTK calls signals, if I recall correctly, and are more often called event handlers. They're ways to register a set of callbacks on a single emission point.

Don’t know about Windows but POSIX and Carbon were as OO as they needed to be - a function where the first parameter is a “file” or “context” is being object oriented.

Honestly I don't know of anyone, including the people who defined those APIs, who would call them object oriented. POSIX functions aren't even namespaced and routinely don't even take a struct as the first parameter. Implicit state is everywhere in POSIX, Win32 and I think also Carbon.

Cocoa succeeded by using messaging and composition instead like Alan Kay wanted.

Cocoa "succeeded" by being the official API of the first non-failed attempt at resurrecting macOS. On Windows, the API that "succeeded" was .NET which is a classical C++ type of OO. In practice, Objective-C caused a lot of problems for Apple. They would have junked it years earlier but Steve Jobs was wedded to it and couldn't understand why anyone wanted anything else. The moment Jobs died Swift was started as a project, and Swift does not have objc_msgSend at its centre, at least not if I understood the Swift ABI correctly.

The "messaging" concept caused problems in the following ways:

* Performance, despite objc message sending being heavily optimised.

* Bizarre semantics that led directly to horrible bugs, like sending a message to null being "valid" but returning junk if the return value was meant to be a struct.

* Difficult to optimise due to the dynamic language features, which are nonetheless hardly used (e.g. ability to redefine methods on the fly).

The latter problem is shared with languages like Ruby.