I'm not that sure about macOS, but on Windows that seems to be way more prevalent in my experience (had to deal with it when worked on clipboard redirection via RDP). The format also has some additional metadata embedded into it [1]. After all, Windows applications kinda pioneered "rich text" exchange via clipboard.
The format is actually negotiated by the communicating applications so it quite can be RTF too, depending on what formats the applications can produce and understand and the priorities they give to them.
I'm not that sure about macOS, but on Windows that seems to be way more prevalent in my experience (had to deal with it when worked on clipboard redirection via RDP). The format also has some additional metadata embedded into it [1]. After all, Windows applications kinda pioneered "rich text" exchange via clipboard.
[1]: https://docs.microsoft.com/en-us/windows/win32/dataxchg/html...
The format is actually negotiated by the communicating applications so it quite can be RTF too, depending on what formats the applications can produce and understand and the priorities they give to them.