But then at some point I think that might have been replaced in practice with HTML as a clipboard format instead.
I'm not sure if you copy text from browsers or word processors today, if it's usually available as both RTF and HTML, or only one?
In fact rtf usage is all over the place on Apple platforms. Cocoa/AppKit apps can take an rtf file to present in the app’s standard About window for credits/acknowledgments and if you want to present rich text with low hassle and resource consumption, by far the easiest way is to embed an NSTextView/UITextView with editing disabled which can then have a bundled rtf loaded into it trivially (just a couple lines of code).
A WebKit webview and HTML could be used instead in these instances, but that’s significantly more heavy and doesn’t have an OS-included WYSIWYG editor which makes it less friendly to e.g. non-technical team members whose job it might be to prepare the content being displayed.