The cable pull ratio is going to be incorrect and you’d need to abuse the limit screws to prevent overshifting. I believe the newer RD’s have slightly different geometry too, which would further compound the issues you’ll run into.
You could put this together (11sp shifter, 9sp cassette and 9sp RD) and it would work after a fashion, but it wouldn’t offer a particularly indexed shift in the normal sense off the term, it would offer really quite poor shift quality – it would be something you’d possibly want to do to get out of a hole but probably not something to do of choice.
