Disclaimer: this is just my personal views, so I may be missing something here.
'unknown' is almost like saying "We don't know what should be here", while 'none' is closer to saying "We didn't get passed any information here".
Ultimately, I feel like 'unknown' is more specific, it conveys intent beyond that of 'none'. Such as, it could imply we actually don't know what kind of data 'Opinions' should hold, rather than that there merely happens to be no data to put there.
We can throw out options for a long time, as there is no perfect choice. Ren went with `none` as it is standard Rebol and Rebol's designer thought about it long and hard.
'unknown' is almost like saying "We don't know what should be here", while 'none' is closer to saying "We didn't get passed any information here".
Ultimately, I feel like 'unknown' is more specific, it conveys intent beyond that of 'none'. Such as, it could imply we actually don't know what kind of data 'Opinions' should hold, rather than that there merely happens to be no data to put there.